Potent Calcium Channel Blocker Research Articles

Diuretic effect induced by intrarenal infusion of nisoldipine in anaesthetized dogs.

1. Renal effects of nisoldipine, a potent calcium channel blocker, were examined in anaesthetized dogs. 2. Intrarenal arterial infusion of nisoldipine (10, 50 ng/kg per min) did not influence mean systemic blood pressure and renal haemodynamics, but did cause dose-related increases in urine flow and urinary excretion of electrolytes. During nisoldipine infusion at the higher dose, the levels of fractional excretion of sodium, potassium and chloride were significantly elevated. 3. In the lithium clearance study, fractional lithium excretion, an index of fractional sodium excretion in the proximal tubules, was unchanged even with the higher dose of nisoldipine, whereas fractional sodium excretion in the distal tubules markedly increased. 4. Nisoldipine elicited a significant increment in osmolar clearance without altering the reabsorption of free water. 5. Thus, intrarenal administration of nisoldipine at a non-hypotensive dose to anaesthetized dogs produced a significant diuretic effect without alteration in renal haemodynamics. Results in lithium clearance and free water clearance studies suggest that nisoldipine exerts a diuretic action by inhibiting sodium transport at the distal nephron segments beyond the proximal tubules.

Inhibitory Effect of Nisoldipine on Angiotensin-II-Induced Renal Actions in Anesthetized Dogs

Renal effects of nisoldipine, a potent calcium channel blocker, were examined in anesthetized dogs. Intrarenal arterial infusion of nisoldipine (2, 10, and 50 ng/kg per min) had no effect on mean systemic blood pressure and heart rate and there was no significant change in renal hemodynamics during infusion of various doses of the drug. Urine flow and urinary excretions of sodium, chloride, and potassium were increased by nisoldipine in a dose-related manner. Fractional excretions of sodium and chloride were markedly elevated with the highest dose given thereby indicating that tubular reabsorptions of sodium and chloride were inhibited by nisoldipine. Nisoldipine (50 ng/kg per min) abolished the decreasing effects of angiotensin-II on glomerular filtration rate, urine flow, and urinary excretion of electrolytes but not the decrease in renal blood flow by the peptide. Angiotensin-II-induced reduction of fractional excretion of electrolytes was completely blocked by nisoldipine. Renal responses to norepinephrine were unaffected by nisoldipine. Thus, nisoldipine administered intrarenally to anesthetized dogs exerts a diuretic action by way of tubular effects, as is the case with other dihydropyridine calcium channel blockers. Nisoldipine seems to effectively antagonize the renal response to angiotensin-II. Thus, the preferential inhibition of angiotensin-II-induced antidiuresis may mean that nisoldipine interferes with stimulatory effects of angiotensin-II on the renal tubular reabsorption of electrolytes and water.

High affinity binding of the calcium channel blocker, (+)-[methyl- 3H]PN200-110( 3HPN) in rat brain

PN200-110 is a recently introduced 1,4-dihydropyridine which has been demonstrated to be a potent calcium channel blocker. 3HPN has been shown to bind in a specific saturable manner to P 2 fractions obtained from brain homogenates from male Sprague-Dawley rats. 3HPN binding was found to be temperature-dependent. Specific 3HPN binding was maximal at 25°C; binding decreased at 2°C and 37°C. The K D calculated from Scatchard analysis was 0.0943±0.0038 nM while the B max was found to be 109.1±2.3 fmol/mg protein. A concentration dependent inhibition of 3HPN binding by various cations was determined and found to be as follows: ZN 2+ > La 3+ > Rh 3+, Al 3+ > Co 2+, Ni 2+, Mn 2+ > Ca 2+, Mg 2+ > Ba 2+ > Sr 2+. These results provide evidence for the existence of central high affinity dihydropyridine receptor sites in rat brain.

A dihydropyridine calcium channel blocker with phosphodiesterase inhibitory activity: Effects on cultured vascular smooth muscle and cultured heart cells

To define the cellular mechanism of action of a dihydropyridine Ca channel antagonist in an experimental model system devoid of neural influences and reflex effects, we studied the actions of RS93522 on cultured vascular smooth muscle cells and on cultured chick embryo ventricular cells. 45Ca uptake by monolayer cultures of vascular smooth muscle cells was inhibited in a concentration-dependent manner. The IC 50 for this effect was 10 n m, similar to that for nifedipine (7 n m) in the same system. 10 −6 m RS93522 inhibited 45Ca uptake more fully than 10 −6 m nifedipine ( P < 0.05). Using an optical-video system, the effect of RS93522 on amplitude of contraction of spontaneously beating cultured ventricular cells was studied. Amplitude of contraction was inhibited with IC 50 = 7.9 × 10 −8 m. 45Ca uptake in myocytes was depressed by 15% at 5 min. RS93522 had the additional property of inhibiting phosphodiesterase activity in myocardial homogenates with IC 50 = 1.6 × 10 −5 m; the potency and efficacy of phosphodiesterase inhibition was similar to that for milrinone in the same system. As expected of Ca channel antagonists, it has a negative inotropic effect on cultured myocardial cells. The compound also has phosphodiesterase inhibitory activity that possibly may potentiate vasodilatation and ameliorate, in part, negative inotropic effects. Thus, RS93522 has two distinct pharmacodynamic effects in myocytes and is a potent calcium channel blocker.

Comparative Binding of Two Closely Related Dihydropyridines (Isradipine and Darodipine) to Serum Proteins and Erythrocytes

The binding of the two drugs isradipine and darodipine, chemically related to dihydropyridines and potent calcium channel blockers, was studied in vitro to isolated plasma proteins, erythrocytes and human serum. The two drugs were strongly bound to serum proteins (up to 97%), mainly to human serum albumin (HSA), alpha 1-glycoprotein (AAG) and lipoproteins (VLDL, LDL and HDL). Their bindings to AAG were saturable with high affinity constants (isradipine 498,000 M-1, darodipine = 155,000 M-1; n = 1). The binding of these drugs to HSA, VLDL and HDL was unsaturable, but it was saturable on LDL. In blood the drugs partitioned in erythrocytes, 16% for isradipine and 14.8% for darodipine.

The effect of inhaled nifedipine on bronchial reactivity to histamine in man

We have administered nifedipine by aerosol to six patients with mild asthma to determine whether local administration of a potent calcium channel blocker has any effect on resting airway tone or histamine reactivity. Subjects had their responsiveness to histamine measured and then received either nifedipine, 10 mg in 40% ethanol, or diluent alone in a randomized, double-blind fashion. Specific airway conductance, blood pressure, and heart rate did not change after either inhalation. Histamine reactivity was significantly reduced after the nifedipine aerosol, the geometric mean provocative concentration causing a 35% fall in specific airway conductance, rising from 5.0 to 10.9 mg/ml of histamine ( p < 0.05). In individuals this protective effect was variable but overall was no greater than that observed after sublingual nifedipine. Plasma nifedipine concentrations were measured in two subjects after administration of the aerosol and confirmed that inhaled nifedipine is absorbed across the bronchial mucosa.

DOPAMINE REGULATION OF GLOMERULAR FILTRATION RATE AND RENAL BLOOD FLOW IN YOUNG AND ADULT RAT.THE POSSIBLE ROLE OF DA2 RECEPTORS

Dopamine (DA),widely used for treatment of acute prerenal failure (APRF),induces renal vasodilation and increase in glomerular filtration rate (GFR).The exact mechanism of DA action has not been clarified.Two types of DA receptors (1 and 2) have recently been identified.DA1 is linked to adenylate cyclase activation,while DA2 is not, and might be linked to calcium. DA1 is predominantly found in the nonglomerular renal vessels,while in the glomeruli only DA2 has been demonstrated. DA has successfully been used for treatment of APRF in preterm infants, but little is known about developmental differences in the renal response to the drug.We examined the renal hemodynamic effects of low dose (0.2 and 1.0/ug/kg/min) DA in young (Y, 24 d old) and adult (A, 60 d old) rats, and also tried to evaluate the possibility of calcium linkage and the role of the DA2 receptors by performing studies with and without verapamil (a potent calcium channel blocker,VP,20/ug/kg/min) infusions.Single nephron (SN) GFR was measured in 10 Y and in 10 A rats. In A rats 0.2 DA induced a 32% (p < 0.01) and 1.0 DA a 45% (p < 0.001) increase in SNGFR. In Y rats the increase of SNGFR was somewhat smaller, but with 1.0 DA it was statistically significant (33%,p < 0.001). In 8 adult rats the addition of VP abolished the DA induced increase in SNGFR.Blood flows (BF) were determined in 32 A rats with microsphere technique. 1.0 DA alone significantly increased renal cortical, renal medullary and mesenterial BFs. The addition of VP significantly decreased the DA induced increase of the renal cortical BF, but did not influence the DA induced vasodilation in the renal medulla and mesenterium. Conclusions: Y rats can respond to low dose DA but certain dose dependent differences might exist between Y and A rats suggesting a maturational process for DA receptors and/or effector responses. At 1.0 DA the drug induced increase of renal cortical BF is mainly modulated by calcium linked receptors,which we assume are DA2s, while in the renal medulla and mesenterium the cAMP linked DA1 plays the main regulating role.

3H]Nitrendipine binding to calcium channels in bovine and rat pituitary.

[3H]Nitrendipine was used to label sites in homogenates of bovine anterior and neurointermediate lobes of the pituitary gland. The amount of specific binding in the anterior lobe was 1.82 +/- 0.30 pmol/g wet weight of tissue and the KD was 1.44 +/- 0.02 X 10(-10) M. Preliminary experiments indicated a similar amount of binding in bovine neurointermediate lobe. In competition studies nimodipine and nisoldipine (two potent voltage-sensitive calcium channel blockers) displayed IC50 values of 1.6 and 6.8 X 10(-10) M, respectively. Verapamil and the verapamil-like calcium channel blockers D-600 and tiapamil competed in a complex manner for the [3H]nitrendipine specific binding to bovine anterior pituitary homogenates. Autoradiographical studies demonstrated specific [3H]nitrendipine binding sites distributed approximately equally in the anterior and posterior lobes, but not in the intermediate lobe of the rat pituitary. In general the properties of [3H]nitrendipine binding in the pituitary tissue resemble strongly the properties of [3H]nitrendipine binding in the brain which is believed to be to voltage-sensitive calcium channels. These results provide support for the hypothesis that calcium channels are involved in pituitary hormone secretion and that drugs that interact with calcium channels may modulate the secretory process directly at the level of the pituitary.

Verapamil attenuates stress-induced gastric ulceration

Since many of the proposed etiologic factors leading to gastric stress ulceration involve stimulation of calcium influx, the effect of verapamil, a potent calcium channel blocker, on the gastric mucosa in cold-restrained inbred rats was assessed. Twenty-nine rats received intraperitoneal normal saline (2 ml) while the experimental group ( N = 29) received 1 mg/kg verapamil in an equal volume of normal saline intraperitoneally. All animals were then stressed at 4°C for 4 hr and sacrificed. Gastrin and fatty acid levels were measured and blinded ulcer scoring of the gastric mucosa was carried out. Verapamil-treated animals had decreased frequency and severity of gastric stress ulceration as assessed by ulcer index, ulcer grade, and number of ulcers/animal. In addition, the plasma gastrin levels tended to be lower in the verapamil group. Fatty acid levels were similarly depressed following cold restraint in both groups. Pretreatment with verapamil significantly decreased gastric ulcerative response to cold-restraint stress in the rat. This effect of verapamil pretreatment may be secondary to cytoprotection of the gastric mucosa, preservation of gastric mucosal blood flow, or blockade of calcium-mediated ulcerogenic stimuli.

Dihydropyridines as potent calcium channel blockers in neuronal cells

Nicardipine, one of the dihydropyridine derivatives, in a nanomolar concentration range suppressed the high K +-induced neurotransmitter from cultured neuronal cells (chick embryonic neural retina cells and clonal rat pheochromocytoma cells). The high K +-induced Ca 2+ uptake into pheochromocytoma cell was also blocked by nicardipine in the same concentration range. [ 3H]Nitrendipine, another dihydropyridine derivative, bound specifically to pheochromocytoma cell homogenate in a saturable manner. We concluded that dihydropyridines block and bind to the high K +-sensitive Ca 2+ channels in neuronal cells.

Evidence of multiple receptor sites within the putative calcium channel.

[3H]-Nimodipine a potent calcium channel blocker, binds to an apparently homogenous population of receptors in guinea-pig brain membranes (KD=0.62 nM, Hill coefficient or approximately 1.0). Diltiazem (10(-5) M) lowers the KD for [3H]-nimodipine by a factor of 3 without changing the maximum number of binding sites. Diltiazem decreased the dissociation rate constant of the nimodipine-receptor complex from 0.18 min-1 to 0.049 min-1 and altered the pharmacological profile as revealed by displacement studies with (-) and (+) verapamil, (-) and (+) prenylamine and 1,4 dihydropyridines. In conclusion [3H]-nimodipine binding can be utilized as a tool to evaluate complex molecular interactions between calcium channel blockers.

The beneficial effect of a calcium channel blocker, diltiazem, on the ischemic-reperfused heart

Diltiazem-HCl, ® a potent calcium channel blocker, was found to be effective in moderating the harmful effects of reperfusion on the severely ischemic myocardium. In isolated working rat heart preparations it was found that 120 min of global ischemia followed by 15 min of reperfusion resulted in a massive leakage of creatine phosphokinase into the coronary effluent, and in many cases, in fibrillation and/or contracture. Left ventricular end-diastolic pressure increased sharply during reperfusion in these hearts. Reperfusion did not affect tissue ATP levels, but did increase creatine phosphate somewhat. When Diltiazem was added to the perfusate (final concentration 0.4 μ m) 5 min prior to re-establishment of flow, the deleterious effects of reperfusion were greatly reduced. None of the hearts fibrillated on reperfusion, and none developed contracture. Left-ventricular end-diastolic pressure was increased only slightly in these hearts. The amount of creatine phosphokinase released into the coronary effluent during re-flow was only one-half that which was released by hearts reperfused in the absence of Diltiazem. In the Diltiazem-treated hearts reperfusion restored creatine phosphate to near-normal levels, although ATP levels were not increased. The beneficial effects of Diltiazem are probably related to its ability to reduce the rapid and massive mitochondrial calcium overloading which normally accompanies reperfusion of severely ischemic myocardium.