Calcium Entry Blocker Verapamil Research Articles

Signal transduction mechanisms involved in in vitro ram sperm capacitation

We validate the chlortetracycline (CTC) technique for the evaluation of capacitation and acrosome reaction-like changes in ram sperm, carrying out a double estimation of the acrosome status after treatment with lysophosphatidylcholine, using fluorescein isocyanate (FITC)-RCA/ethidium homodimer 1 (EthD-1) and CTC/EthD-1. Highly consistent results and a positive correlation between the results of acrosome-reacted sperm evaluated with both techniques were obtained. In this study, we evaluate the effects of ram sperm capacitation of BSA, Ca(2+), NaHCO(3) and cAMP agonists and their influence on the associated protein tyrosine phosphorylation. We found a time-dependent increase in capacitation related to protein tyrosine phosphorylation, either in the absence or the presence of BSA. The addition of an increasing concentration of cholesterol to samples containing BSA did not influence results. The effect of bicarbonate was concentration-dependent, with a significantly lowered value of non-capacitated sperm in the presence 18 and 25 mM. The addition of extracellular calcium did not significantly increase either the proportion of capacitated sperm or the protein tyrosine phosphorylation signalling, although a significantly higher value of acrosome-reacted sperm was found in samples containing 4 mM Ca(2+). cAMP agonists increased capacitated sperm and protein tyrosine phosphorylation signalling. The inhibition of protein kinase A by H-89 caused a decrease in sperm capacitation. Addition of a calcium-entry blocker (Verapamil; Sigma) did not influence results, which suggests that the calcium entry blocker was unable to inhibit the calcium influx associated with capacitation in ram sperm. Our findings might benefit our understanding of the biochemical mechanisms involved in mammalian sperm capacitation and ultimately, fertility.

Effects of nifedipine, verapamil and diltiazem on serum biochemical parameters and aortic composition of atherosclerotic chickens

Calcium appears to be involved in many of the cellular events, which are thought to be important in atherogenesis. In this study, we examine the effects of three calcium entry blockers (nifedipine, verapamil, and diltiazem at clinical and higher doses) on serum biochemical parameters and aortic calcium, cholesterol and triglyceride concentrations of atherosclerotic egg-fed chickens. All egg-fed chickens (treated and non-treated) showed an increase in serum total cholesterol, LDL-cholesterol and triglycerides without significant effect when calcium entry blockers were used. Increased HDL values were observed in clinical and high-dose nifedipine and clinical dose verapamil groups. The high-dose diltiazem group presented increased zinc values with respect to the clinical dose diltiazem and control groups. The sodium concentrations were significantly decreased in all the groups of animals treated with calcium entry blockers at high-doses and nifedipine at clinical doses. Measurements of aortic calcium concentration showed a significant decrease in the high-dose nifedipine and verapamil groups. Calcium channel blockers had a tendency to decrease total cholesterol in aortas. The values were statistically significant for the high-dose verapamil, and nifedipine groups. Only nifedipine showed a significant decrease for this parameter at clinical dosages. Triglyceride concentrations in aortas were significantly low in animals fed an atherogenic diet and treated with calcium channel blockers, without differences between drugs or dosages used in the experiment. In addition, the chicken atherosclerosis model has proved itself useful and very suitable for in vivo drug intervention studies.

Arterial structural changes with verapamil in spontaneously hypertensive rats

Reducing pulse pressure might be more powerful than reducing mean arterial pressure to obtain regression of vascular hypertrophy. However, this hypothesis has never been investigated in the conduit arteries of intact hypertensive animals. A group of 4-week-old spontaneously hypertensive rats (SHR) was treated with the calcium-entry blocker verapamil (50 mg/kg) for 16 weeks and compared with untreated SHR and control Wistar Kyoto (WKY) normotensive rats of the same age. At the end of the experiment, intraarterial thoracic aorta blood pressure was measured both in the conscious and anesthetized animals. Carotid artery diameter and stiffness (echo-tracking techniques) and aortic histomorphometry were determined in parallel. With verapamil, pulse pressure, but not mean arterial pressure, was significantly decreased but did not reach the normotensive values. Carotid internal diameter, medial thickness, and collagen content were significantly reduced by comparison with SHR and did not differ from the values of the WKY group. A significant positive and independent correlation was observed between pulse pressure and medial thickness in the overall population. The study shows that, in SHR chronically treated with verapamil, structural changes may be completely prevented without any change in mean arterial pressure. The parallel change in pulse pressure might suggest that mechanosensitive elements within the vascular wall may be selectively sensitive to the dynamic aspects of physical forces and are able to convert frequency and amplitude information into cellular responses that lead to vascular remodeling.

Inhibition of calcium entry preserves contractility of arterial smooth muscle in culture.

The addition of the growth stimulator fetal calf serum (FCS, 10%) to rings of rat tail artery causes an increase in [Ca2+]i, accompanied by contraction. This response was inhibited by the calcium entry blocker verapamil (1 microM). To investigate the effect of Ca2+ entry blockade on growth and contractility, rings of rat tail artery were cultured for 4 days in medium with or without FCS and then mounted for tension registration and stimulated with noradrenaline (NA) or high-K+ solution. In cultured rings growth was quantitated by [3H]-thymidine incorporation and increase in protein contents. FCS in the medium stimulated DNA synthesis by about 2-fold and increased protein contents by about 70%. The growth-stimulated cultured rings developed less force than freshly prepared rings (2.2 +/- 0.3 vs. 8.3 +/- 1.0 mN/mm). The addition of 1 microM verapamil to the medium during culture increased maximal NA-evoked force to 5.0 +/- 0.4 mN/mm but had no effect on the increases in DNA synthesis and protein contents. Force developed by growth-arrested rings, cultured in the absence of FCS, was not different from that of freshly prepared rings (7.2 +/- 0.6 mM/mm). Verapamil did not affect maximal force in these rings. Similar responses were seen when contraction was elicited by high-K+ solution. We conclude that verapamil, present during culture, preserves contractility of arterial smooth muscle, and that this effect is not parallel to inhibition of growth.

Effect of verapamil on intimal thickening and vascular reactivity in the collared carotid artery of the rabbit.

1. Intimal thickening is a common site for atherosclerosis. Therefore, we investigated whether the calcium entry blocker verapamil (10 mg kg-1 body weight day-1, s.c.) can retard intimal thickening and changes in vascular reactivity induced by a non-occlusive, silicone collar positioned around the left carotid artery of rabbits. The contralateral carotid artery was sham-operated and served as a control. 2. Verapamil and placebo (saline 0.1 ml kg-1, day-1, s.c.) treatments were initiated 7 days before placing the collar and lasted 3 weeks. Thereafter, segments were cut from collared and sham-treated arteries for histology and isometric tension recording. 3. The intima/media (I/M ratio increased after 14 days of collar treatment, but intimal thickening was not inhibited by verapamil (I/M ratio placebo 0.31 +/- 0.07, verapamil 0.32 +/- 0.09). 4. The collar decreased the capacity to develop force, as indicated by the response to a supramaximal concentration of KCl, decreased the sensitivity (pD2) to acetylcholine (ACh) and phenylephrine (Phe), but increased the sensitivity to 5-hydroxytryamine (5-HT). 5. Although verapamil did not affect intimal thickening, it normalized the hypersensitivity to 5-HT in collared arteries. 6. The contraction to the supramaximal concentration of KCl was not affected by verapamil. Verapamil decreased the Emax of ACh, but this was only seen in collar-treated arteries. Verapamil also decreased the sensitivity to ACh and Phe, in both sham- and collar-treated arteries. 7. We conclude that verapamil, without preventing thickening of the intima, can modify collar-induced changes in vascular reactivity.

Verapamil and flunarizine protect the isolated perfused rat liver against warm ischemia and reperfusion injury.

Using the model of the isolated perfused rat liver, we investigated the influence of the two pharmacologically different calcium channel blockers, verapamil and flunarizine, on changes of ion homeostasis, liver weights, pH deviations and enzyme activities during warm ischemia (37 degrees C) and reperfusion. The LDH and GLDH activities were determined and the calcium, potassium, and sodium concentrations were measured in the effluent. Warm ischemia (180 min) caused an increased enzyme release, a high influx of calcium and sodium into the liver and a massive potassium efflux current. Normoxic reperfusion led to a further increase in hepatic enzyme release and although the loss of potassium ceased, the calcium influx into the liver continued. By the end of reperfusion the liver weight had increased significantly (P < 0.01) in the control group. The two calcium entry blockers were added to the perfusate in various concentrations. Both substances protected the liver against warm ischemia and normoxic reperfusion damage, but they did not inhibit calcium inflow. However, the potassium efflux was significantly reduced by all concentration tasted (P < 0.001). After reperfusion the liver weights were significantly lower in the treated groups (P < 0.001) than in control animals. Thus, the calcium entry blockers verapamil and flunarizine protect liver cells against damage caused by warm ischemia and reperfusion. Furthermore, they prevent the disruption of intracellular potassium homeostasis, which seems to be related to improved volume regulation of liver cells.

Protective Effect of Verapamil on Shock Wave Induced Renal Tubular Dysfunction

In a prospective randomized study, the effects of the calcium entry blocker verapamil on shock wave induced tubular impairment were examined. A total of 24 patients with renal pelvis or caliceal stones undergoing anesthesia-free extracorporeal shock wave lithotripsy (ESWL**Dornier Medical Systems, Inc., Marietta, Georgia.) without auxiliary measures was randomly assigned to the verapamil group (12) or the control group (12). Four doses of verapamil (80mg. each) were given orally starting the night before ESWL. Controls received no medication. To assess renal tubular function the urinary excretion of α1-microglobulin, N-acetyl-β5-glucosaminidase and Tamm-Horsfall protein were determined before, immediately, and 12 and 24hours after ESWL. After ESWL there was an increase in urinary α1-microglobulin and N-acetyl-β5-glucosaminidase, which was significantly higher in the control than in the verapamil group. Tamm-Horsfall protein, a glycoprotein synthesized by the distal tubules, decreased significantly less in the verapamil group compared to the controls. Our results indicate that verapamil exhibits a protective effect on shock wave induced tubular damage. The underlying mechanisms are not elucidated yet, and direct actions on tubular cells and interference with renal hemodynamics are to be discussed.

Effect of verapamil on hepatic ischemia/reperfusion injury

Alterations in cellular calcium homeostasis are a critical factor in the pathogenesis of hepatic ischemic damage and may mediate oxygen free radical injury during the reperfusion period. We investigated the effect of the calcium channel blocker verapamil on hepatic ischemia/reperfusion injury in normal rats and rats sensitized to oxidative injury by chemical depletion of the endogenous antioxidant glutathione. Forty-five minutes of complete hepatic ischemia followed by reperfusion caused an increase in the serum glutamic pyruvic transaminase (SGPT) level and a decline in the endogenous hepatic glutathione level but no increase in hepatic lipid peroxidation products. Chemical depletion of hepatic glutathione with diethylmaleate did not induce hepatocellular injury but augmented hepatic ischemia/reperfusion-induced SGPT release and promoted lipid peroxidation. Pretreatment with the calcium entry blocker verapamil protected against the ischemia/reperfusion-induced drop in hepatic glutathione but did not reduce SGPT release in normal rats. In rats sensitized to oxidative injury by chemical depletion of endogenous glutathione, the calcium channel blocker verapamil protected against ischemia/reperfusion-induced lipid peroxidation and reduced the release of SGPT. These findings indicate that the rat liver is protected against oxidative injury after short periods of total ischemia by its rich supply of endogenous glutathione. A beneficial effect of verapamil occurs only in rats sensitized to oxidative injury, suggesting that the calcium channel blocker protects against oxygen radical attack.

Idiopathic Long QT Syndrome Exacerbated by Beta‐Adrenergic Blockade and Responsive to Left Cardiac Sympathetic Denervation: Implications Regarding Electrophysiologic Substrate and Adrenergic Modulation

Electrophysiologic Substrate of Long QT Syndrome. Introduction: The mechanisms of T wave abnormalities and arrhythmias in patients affected by the long QT syndrome (LQTS) are unclear, as are the reasons why a high‐risk subgroup (20%) continues to have syncope during therapy with beta blockers but is protected after left cardiac sympathetic denervation (LCSD). Afterdepolarizations, both early (EAD) or delayed (DAD), have been implicated as likely electrophysiologic substrates for the ECG abnormalities and arrhythmias in LQTS. To test this hypothesis, we analyzed nine Hotter recordings of a LQTS patient typical of the high‐risk subgroup. Methods and Results: We applied to the ECG rules derived from previous in vitro and in vivo studies, which relate the coupling intervals of the dysrhythmic beats to the preceding cycle lengths, to discriminate among arrhythmia mechanisms. In the absence of therapy, the patient's ECG showed peaked T waves, with a notched second component accentuated by sinus pauses. The amplitude of the notches increased directly with the preceding RR interval (r = 0.58, P &lt; 0.05), as it is observed with EAD. Therapy with the beta blocker propranolol did not modify the notches, which maintained their amplitude and their relationship with the preceding RR (r = 0.62, P &lt; 0.05). Moreover, during beta‐blocker therapy, ventricular premature beats, couplets, and runs of ventricular tachycardia (VT) occurred. The coupling intervals of ventricular premature beats and couplets were not influenced by the preceding RR intervals. By contrast, the coupling interval of the first beat of VT decreased as the preceding RR shortened, as it occurs with DAD‐induced triggered activity. After LCSD, T wave notch amplitude was reduced (P &lt; 0.05) and was no longer influenced by the preceding RR (r = 0.31, P = 0.22). The same was true when the calcium entry blocker verapamil was administered. Conclusion: In this patient, T wave abnormalities and some ventricular arrhythmias had the behavior observed in vitro for EAD and the consequent triggered activity. However, the cycle length dependence of VT manifested the behavior observed in vitro for DAD and the related triggered activity. The fact that these phenomena were accentuated by beta blockade and disappeared after LCSD suggests that the alpha‐adrenergic receptor‐effector system may be a modulator of T wave abnormalities and arrhythmias in some patients unresponsive to beta blockade and protected by LCSD. (J Cardiovasc Electrophysiol, Vol. 3, pp. 295–305, August 1992)

Interaction of prostaglandins with adenosine diphosphate induced increase in cytosolic free calcium in human platelets.

The interaction of prostaglandins with changes in cytosolic Ca2+ concentration ([Ca2+]) and aggregation of human platelets induced by adenosine diphosphate (ADP) were investigated. Cytosolic [Ca2+] was measured with the fluorescent dye Quin2. Addition of ADP (0.25-2.5 mumol l-1) to platelet suspensions produced a dose dependent increase in cytosolic [Ca2+] from a basal level of 51 +/- 1 nmol l-1 to maximum levels exceeding 1 mumol l-1 and induced platelet aggregation. Chelation of extracellular calcium with 100 mumol l-1 EGTA markedly reduced the increase in cytosolic [Ca2+] induced by 0.25 mumol l-1 ADP, while pretreatment with the calcium entry blocker verapamil was without effect. Stimulation of cyclic AMP with prostaglandins (PGD2, PGE1, PGE2, PGI2, but not PGF2 alpha) and forskolin, or incubation with dibutyryl-cAMP, inhibited the rise in cytosolic [Ca2+] and platelet aggregation following ADP. We conclude that prostaglandins inhibit the increase in cytosolic [Ca2+] and aggregation of human platelets induced by ADP, probably by stimulation of cyclic AMP generation, thereby opposing the mechanism by which ADP increases cytosolic [Ca2+] and subsequently induces platelet aggregation.

Increased toxicity of anthracycline antibiotics induced by calcium entry blockers in cultured cardiomyocytes

Calcium channel blocking drugs have been reported to reduce survival rate of laboratory animals treated with cardiotoxic antitumor anthracyclines. In order to elucidate the mechanisms of this drug interaction, cell toxicity of the anthracyclines, doxorubicin and daunorubicin, was evaluated in primary cultures of cardiac myocytes isolated from neonatal rats. Low concentrations of extracellular calcium ([Ca 2+] 0) and addition of calcium entry blockers (nifedipine or flunarizine) potentiated myocardial toxicity of anthracyclines as assessed by the release of lactate dehydrogenase from the cells. Accumulation of anthracyclines in the cardiomyocytes was increased by calcium entry blockers (nifedipine, flunarizine, and verapamil) and by low [Ca 2+] 0; efflux of [ 3H]daunorubicin from myocardial cells was inhibited by nifedipine. At a dose that exerts only modest calcium channel activity, R-verapamil failed to affect doxorubicin accumulation in cardiomyocytes, whereas the calcium channel activator, (±)-Bay K-8644, reduced the retention of anthracyclines; the calcium channel activity is thus required in order to increase the accumulation of anthracyclines in myocardial cells. Calcium channel blockers are also known to increase intracellular retention and toxicity of chemotherapeutic drugs in multidrug resistant tumor cells by inhibiting the efflux of cytotoxic agents from cells; however, the ability of the interacting drugs to inhibit the efflux of chemotherapeutic agents from tumor cells is not dependent on the calcium channel blocking activity. Therefore, the mechanism(s) by which calcium channel blocking drugs increase the accumulation of anthracyclines in resistant tumor cells and myocardial cells may be different. In accordance with previous investigations, the present in vitro study confirmed that anthracycline-induced cardiotoxicity may be potentiated by calcium channel blocking drugs. This indicates that, in the association of antineoplastic drugs with agents that reverse multidrug resistance, the potential exists for enhanced damage of normal cells and tissues; further studies are needed to evaluate the relevance of this adverse interaction.

Combined negative inotropic effects of calcium entry blockers and isoflurane on canine isolated heart muscles.

The combined negative inotropic effects of isoflurane and calcium entry blockers (verapamil, diltiazem, nifedipine, nicardipine) were studied utilizing isolated heart preparations of ventricular muscles from dogs. All of these calcium entry blockers exerted dose-dependent decreases in maximal velocity of shortening (Vmax), maximal developed isometric force (Fm), and the maximal first derivative of Fm (maximal dF/dt). Dose-dependent decreases of these variables of muscle mechanics were augmented in isoflurane-depressed myocardium. At equimolar concentrations, direct myocardial depression was demonstrated in the following order of severity: nifedipine > diltiazem = verapamil > nicardipine. Percent depressions of Vmax, Fm and maximal dF/dt were significantly greater in muscles when calcium entry blockers were combined with 1MAC isoflurane than in muscles of calcium entry blockers alone. These data suggest that the negative inotropic effects of verapamil, diltiazem, nifedipine, and nicardipine were potentiated by isoflurane.

Effect of verapamil on conversion of xanthine dehydrogenase to oxidase in ischemic rat liver

The effect of organ flushing with the calcium entry blocker verapamil on the conversion of innocent enzyme xanthine dehydrogenase (XDH) to superoxide generating enzyme xanthine oxidase (XOD) in ischemic rat livers was studied. This enzyme conversion progressed over time in warm or cold ischemia. In non-flushed livers, the activities of XOD as percentages of XDH plus XOD after 6 h at 37 degrees C and 6 days at 4 degrees C were 80.3 +/- 5.2 and 31.6 +/- 2.1, respectively. In the livers flushed with Euro-Collins solution, the conversion was inhibited to 37.0 +/- 3.9% (P less than 0.001) after 6 h of warm ischemia, while this inhibitory effect was not found in cold ischemia. Verapamil given through the portal vein on flushing further suppressed the conversion in both warm and cold ischemia (with 5.0 microM of verapamil, 21.2 +/- 5.8% (P less than 0.001) after 6 h of warm ischemia and 25.2 +/- 3.3% (P less than 0.01) after 6 days of cold ischemia). A similar effect was also obtained with the addition of 10 or 30 mM of EGTA instead of verapamil. In contrast, no inhibitory effect on conversion was obtained in livers flushed and homogenized with 10.0 microM of verapamil followed by incubation for 6 h at 37 degrees C. In the livers that were flushed and stored at a warm temperature for 6 h, verapamil reduced the increase of tissue lipid peroxidation product (P less than 0.02) after 15 min of reperfusion. Although the precise mechanisms of these inhibitory effects of verapamil on the enzyme conversion are still uncertain, it is thought that organ flushing with verapamil might reduce the XOD-mediated postischemic reperfusion injury in livers subjected to prolonged ischemia.

Influence of verapamil on regional renal blood flow: a study using multichannel laser‐Doppler flowmetry

In anaesthetized male Sprague-Dawley rats the influence of the calcium entry blocker verapamil (2.4 mg h-1 kg-1, i.v.) on renal superficial cortical (CO) and outer medullary (OM) blood flow was investigated with the aim of elucidating further the intrarenal heterogeneity in vasoreactivity. The blood flow of the two regions was monitored simultaneously with a laser-Doppler flowmeter, using fibre probes with an outer diameter of 0.75 mm. One probe was directed towards the cortex and a second probe was inserted through the cortex and positioned in the outer medulla, measuring the flow within a hemisphere with a depth of 0.5-1.0 mm. The OM probe position was verified by dissecting the kidneys after each experiment. Insertion of the OM probe did not affect whole-kidney glomerular filtration rate, renal plasma flow or electrolyte excretion. Thirty minutes of verapamil infusion increased OM blood flow by 26%, but did not change CO blood flow (-1%). In spite of the increase in OM blood flow, the urine osmolality remained unaltered. Sodium excretion increased by 39%, while potassium excretion was unchanged. Mean arterial blood pressure decreased by 13%. In conclusion, this study has further supported the suggestion that the vasoreactivity is higher in the juxtamedullary than in the superficial cortical vasculature. The heterogeneity of the response is most probably due to the previously documented pressure drop along the interlobular arteries, which will create different haemodynamic conditions for the juxtamedullary and superficial afferent arterioles.

Effects of Verapamil on Diurnal and Thyrotropin-Releasing Hormone-Stimulated Prolactin Levels in Man

The effects of the calcium entry blocker verapamil on the 24-h profile of PRL secretion and on the PRL response to TRH were investigated in six healthy volunteers. Verapamil (120 mg, three times daily) was administered orally for 1 week. In all subjects both basal and TRH-stimulated PRL levels were markedly elevated by verapamil. The average diurnal PRL concentration was increased from 13.0 +/- 2.0 micrograms/L to 25.2 +/- 4.4 (mean +/- SE; P = 0.02). Diurnal rhythm and pulsatility of PRL secretion were seen both before and during verapamil administration. Mean peak PRL concentrations after TRH injection (200 micrograms, iv) were significantly increased from 72.6 +/- 11.6 to 115.2 +/- 16.8 (P less than 0.01), and the mean area under the PRL concentration-time curves from 4332 +/- 962 micrograms/L.120 min to 6975 +/- 1334 (P = 0.01). The data are in striking contrast with previous findings from in vitro studies where verapamil has been reported to block calcium-mediated stimulus-secretion coupling and inhibit hormone secretion from pituitary cells. Interference with other PRL-regulating mechanisms may account for the demonstrated verapamil-induced PRL secretion in vivo.

Regulation of renin release and renal hemodynamics during acute and chronic verapamil administration

The effects of short- and long-term administration of the calcium-entry blocker verapamil on regulation of renal hemodynamics and renin release were analyzed in anesthetized dogs at controlled levels of renal perfusion pressure between 110 and 60 mmHg. The following three groups of dogs were studied: a control group, an acutely treated verapamil group (initial dose of 150 micrograms/kg followed by 4 micrograms.kg-1.min-1), and a chronically treated verapamil group (240 mg per os twice each day). At 110 mmHg the renal blood flow (RBF) in the acutely and chronically treated groups was 53% greater than (P less than 0.03) and 79% greater than (P less than 0.01) that of the control group. Glomerular filtration rate (GFR) in the acutely and chronically treated groups was 64 and 92%, respectively, greater than (P less than 0.01 for both) that of the control group. Autoregulation of RBF and GFR was extremely effective in the control group, whereas it was severely impaired by acute and long-term treatment with verapamil. Renin release at 110 mmHg in the control and the acutely treated groups were similar, 3.36 +/- 0.96 and 4.46 +/- 0.64 U.min-1.g-1, respectively, although in the chronically treated group the rate of release was 8.89 +/- 2.60 U.min-1.g-1 (P less than 0.03). As perfusion pressure was reduced, the rate of release from the acutely treated group rose to higher levels than that of the control group; at 70 mmHg release from the acutely treated group was 88% greater than that of the control group (P less than 0.01). Verapamil profoundly alters regulation of renal hemodynamics and renin release and the effect is prominent during both short- and long-term administration.

The synthesis and pharmacological evaluation of indole congeners of the calcium entry blocker verapamil

Based on the notion of a bioisosteric relationship between catechol-(phenol-) and indole-amines, indole congeners of the calcium entry blocker verapamil were synthesized and examined as calcium entry blockers and as alpha 1-adrenoceptor antagonists in isolated tissue preparations and as antihypertensive agents in the spontaneously hypertensive rat. Indole 27 exhibited potent calcium entry blockade in vitro and displayed antihypertensive activity, albeit slightly less than verapamil. Indole 23 possessed both calcium entry blockade and potent alpha 1-adrenoceptor activity in vitro but in vivo was less active than verapamil as an antihypertensive agent.

The effects of verapamil upon noradrenaline-induced contraction of the rat isolated aorta following acute and prolonged alterations in PO2.

1. Noradrenaline (NA;ED90) caused a contraction of the rat aorta which could be separated into two components, a rapid response mediated by release of intracellular Ca2+ and a more slowly developing contraction which relied principally upon Ca2+ influx. 2. Exposure to acute (30 min) hypoxia has been previously shown to reduce the NA-induced contraction (by 28.0 +/- 2.7%, n = 168) which recovered completely upon re-oxygenation (recovery response). In the present study, prolonged exposure to hypoxia (70 h) caused a more pronounced reduction (39.7 +/- 3.0%, n = 90) of the NA-induced contraction, but, re-oxygenation then produced incomplete recovery to 77.9 +/- 3.9% (n = 90) of the control response. 3. Prolonged exposure to 95% O2 caused a 36.5 +/- 3.1% (n = 42) reduction of NA-induced contractions, whereas prolonged exposure to 21% O2 only caused a small (12.6 +/- 3.4%, n = 6) depression of these responses. 4. The component of the NA-induced contraction mediated by release of intracellular Ca2+ is 39.8 +/- 1.3% (n = 83) of the NA contraction in Ca-containing Krebs solution and was previously found to be unaffected by acute hypoxia. However, following prolonged exposure to either hypoxia or 21% O2, this component only reached 30.7 +/- 2.2% (n = 32) or 28.3 +/- 0.9% (n = 6) of the control response, respectively. Prolonged exposure to 95% O2 caused a more pronounced reduction of this component of contraction which then reached 19.1 +/- 2.1% (n = 12) of the control response. 5. Verapamil (10nM-10 microM) produced similar concentration-dependent reductions of NA-induced contractions elicited during control conditions or acute hypoxia; under these conditions, 1 microM verapamil caused a 34.1 + 6.9% (n = 6) and a 41.8 + 2.9% (n = 18) reduction of these responses respectively. However, recovery responses caused by re-oxygenation of tissues exposed to acute hypoxia were more sensitive to verapamil which, at a concentration of 1 microM, caused a 59.2 + 2.7% (n = 18) reduction of these responses. Verapamil (10 nM-10 microM) also caused similar pronounced concentration-dependent reductions of contractions elicited during prolonged exposure to normoxia or hyperoxia and of recovery responses obtained following re-oxygenation of tissues exposed to prolonged hypoxia; 1 microM verapamil caused a 62.5 + 1.1% (n = 6), 77.2 + 3.8% (n = 12) and a 68.0 + 4.3% (n = 12) reduction of these responses respectively. In contrast, contractions elicited during prolonged hypoxia were less sensitive to verapamil which at a concentration of 1 microM only caused a 16.2 + 2.2% (n 12) reduction of these responses. 6. The present study indicates that prolonged exposure of the rat aorta to either hypoxic or oxygenated conditions causes attenuation of NA-induced contraction. However, these effects are also accompanied by changes in tissue Ca2+ handling which differ under each condition and might account for the observed modifications in tissue sensitivity to the calcium-entry blocker verapamil.

The effect of halothane, isoflurane, and verapamil on ischemic-isolated rabbit renal tubules.

The effects of the volatile anesthetics halothane and isoflurane, and the calcium entry blocker verapamil, were studied in isolated rabbit renal tubules under nonischemic and simulated ischemic conditions. Isolated rabbit renal tubules were subjected to zero (control), 30 (I-30), or 60 (I-60) minutes of simulated ischemia following the method of Weinberg. Following the ischemic period, tubules were reoxygenated in a Gilson respirometer (simulated reperfusion) and treated with either halothane (1%) or isoflurane (1%) in the controls and at I-30, or halothane (1%, 2%, 4%) or verapamil (5 microM, 15 microM, 30 microM) at I-60. Tubules were analyzed for lactate dehydrogenase (LDH) release (measuring cell membrane integrity), intracellular potassium and adenosine triphosphate (ATP), and oxygen consumption (cellular respiratory rate). In nonischemic tubules, exposure to 1% isoflurane caused significantly reduced LDH release compared with that released by controls, indicating cell membrane protection, whereas 1% halothane had no effect on these cells. With 30 min of ischemia, 1% isoflurane was associated with significantly higher cellular LDH release and lower ATP concentration, suggesting increased cellular damage. Halothane (1%) was associated with only an increased ATP concentration in tubules exposed to 30 min of ischemia. Following 60 min of ischemia, halothane (4%) decreased LDH release by 45% (29.2 +/- 2.3% vs. 47.0 +/- 9.6% without halothane). Tubules exposed to halothane also had higher intracellular potassium and ATP concentrations, and increased respiratory rates. Halothane (2%) was less protective and only increased the ATP concentration. The release of LDH was not statistically different with or without 2% halothane.(ABSTRACT TRUNCATED AT 250 WORDS)

The calcium entry blocker verapamil increases red cell flux in the vasa recta of the exposed renal papilla.

To investigate the vasoreactivity of the vessels of the juxtamedullary vasculature, measurements were made of the red blood cell flux (Qrbc) in the vasa recta of the exposed renal papilla of anaesthetized rats by means of fluorescently labelled erythrocytes. The flux was measured before and after intravenous administration of the calcium entry blocker verapamil, which was infused in two different doses. A control group receiving vehicle was studied in parallel. The lower dose of verapamil (0.6 mg h-1 kg-1 body wt) increased Qrbc by 30 +/- 6% (n = 9, P less than 0.05, mean +/- 1 SEM) and reduced the systemic blood pressure by 6% (P less than 0.05). The osmolality of urine samples from the papillary tip remained statistically unchanged (1199 +/- 60 vs. 1186 +/- 102 mosM kg-1 H2O). In the animals receiving the higher dose of verapamil (2.4 mg h-1 kg-1 body wt), Qrbc increased by 39 +/- 7% (n = 10, P less than 0.05) and the blood pressure was reduced by 9% (P less than 0.05), while urine osmolality was unchanged (1104 +/- 110 vs. 1264 +/- 123 mosM kg-1 H2O). In the time control group there was no statistically significant change in Qrbc (+6 +/- 8%, n = 10), blood pressure or urine osmolality (1006 +/- 61 vs. 1091 +/- 70 mosM kg-1 H2O). The verapamil-induced increase in Qrbc is probably mainly mediated by a relaxation of the afferent arteriole of the juxtamedullary nephrons. The results confirm our previous indication of a high vasoreactivity in the vessels of the juxtamedullary vasculature.(ABSTRACT TRUNCATED AT 250 WORDS)