8644-induced Contraction Research Articles

Vasorelaxant effect of monoterpene carvacrol on isolated human umbilical artery.

Carvacrol (CRV) is the main compound of essential oils extracted primarily from Thymus and Origanum species. Its various biological activities were confirmed: antioxidant, anti-inflammatory, antibacterial, antifungal, anti-tumour, antinematodal, and vasorelaxant action. Although vasodilation mediated by CRV was previously described, the exact mechanism of its action has not yet been established. Hence, the aim of this study was to investigate CRV vasoactivity on human umbilical arteries (HUA) and the different pathways involved in its mechanism of action using the tissue bath methodology. CRV caused a significant decrease in vascular tension of 5-HT-pre-contracted umbilical arteries, with EC50 of 442.13±33.8µmol/L (mean±standard error of the mean-SEM). At 300µmol/L, CRV shifted downward the 5-HT concentration-response curve with a statistical significance of p<0.001 obtained for the four highest concentrations. At a concentration of 1mmol/L, CRV completely abolished BaCl2-induced contraction in Ca2+-free Krebs-Ringer bicarbonate solution and the BAY K 8644-induced contraction in Krebs-Ringer bicarbonate solution (p<0.001). Isopentenyl pyrophosphate, the antagonist of TRPV3 channel, was able to decrease the efficacy of CRV (p<0.001). The blocking of L-type Ca2+ channels on smooth muscle cells is the most probable mechanism of CRV-induced vasorelaxation. However, the role of TRPV3 channels in CRV-induced vasodilation of HUA cannot be excluded either.

Borneol reduces sympathetic vasomotor hyperactivity and restores depressed baroreflex sensitivity in rats with renovascular hypertension.

Borneol is a bicyclic monoterpene that has long been used in traditional Chinese medicine to increase blood-brain barrier permeability and has shown promising cardiovascular effects. The present study aimed to evaluate the effect of borneol on vascular tone, blood pressure, autonomic function, and baroreflex sensitivity in normotensive and hypertensive rats. A combination of in vitro and in vivo assays was performed in 2-kidneys-1-clip hypertensive rats (2K1C) and their controls (sham). We assessed the in vivo effect of oral treatment with borneol on blood pressure, heart rate, autonomic function, and baroreflex sensitivity in sham and 2K1C rats. Additionally, the vasorelaxant effect of borneol in the superior mesenteric artery isolated from rats and its mechanism of action were evaluated. Oral administration of borneol (125 mg/kg/day) reduced blood pressure, sympathetic vasomotor hyperactivity, and serum oxidative stress and improved baroreflex sensitivity in 2K1C rats. In vessel preparations, borneol induced endothelium-independent vasodilatation after precontraction with phenylephrine or KCl (60 mM). There was no difference in the vascular effect induced by borneol in either the 2K1C or the sham group. In addition, borneol antagonized the contractions induced by CaCl2 and reversed (S)-(-)-Bay K 8644-induced contraction. These data suggest that borneol presents antihypertensive effects in 2K1C rats, which is associated with its ability to improve autonomic impairment and baroreflex dysfunction. The borneol-induced relaxation in the superior mesenteric artery involves L-type Ca2+ channel blockade. This vascular action associated with the antioxidant effect induced by borneol may be responsible, at least in part, for the in vivo effects induced by this monoterpene.

Salvia sclarea L. Essential Oil Extract and Its Antioxidative Phytochemical Sclareol Inhibit Oxytocin-Induced Uterine Hypercontraction Dysmenorrhea Model by Inhibiting the Ca2+-MLCK-MLC20 Signaling Cascade: An Ex Vivo and In Vivo Study.

Salvia sclarea essential oil is used as an aromatic therapy for dysmenorrhea. Sclareol—one of the natural products isolated from S. sclarea—displays anti-inflammatory and antioxidant activities; however, researchers have not yet evaluated the mechanism related to the pain-relieving effect of sclareol. In the present study, we aimed to investigate the potential effect of sclareol in ex vivo and in vivo dysmenorrhea models, as well as its possible mechanism. In the ex vivo study of uterine tissue from Sprague Dawley (SD) rats, the uterine contraction amplitude was observed and recorded. In the in vivo study, we measured the uterine contraction pressure of SD rats and performed writhing tests on mice. The uterine tissues from the writhing test subjects were collected and analyzed by Western blot. The results demonstrated that sclareol inhibited prostaglandin (PG) F2α-, oxytocin-, acetylcholine-, carbachol-, KCl-, and Bay K 8644-induced uterine contraction and possessed an analgesic effect in the writhing test. Sclareol affects the Ca2+ level and regulates oxytocin receptor (OTR), myosin light chain kinase (MLCK), extracellular signal-regulated kinase, p-p38, cyclooxygenase-2 (COX-2), and phospho-myosin light chain 20 (p-MLC20) protein expression. Integrating these results, we suggest that sclareol is a potential alternative supplement for dysmenorrhea.

Cav1.2 channel current block by the PKA inhibitor H-89 in rat tail artery myocytes via a PKA-independent mechanism: Electrophysiological, functional, and molecular docking studies

To characterize the role of cAMP-dependent protein kinase (PKA) in regulating vascular Ca2+ current through Cav1.2 channels [ICa1.2], we have documented a marked capacity of the isoquinoline H-89, widely used as a PKA inhibitor, to reduce current amplitude. We hypothesized that the ICa1.2 inhibitory activity of H-89 was mediated by mechanisms unrelated to PKA inhibition. To support this, an in-depth analysis of H-89 vascular effects on both ICa1.2 and contractility was undertaken by performing whole-cell patch-clamp recordings and functional experiments in rat tail main artery single myocytes and rings, respectively. H-89 inhibited ICa1.2 with a pIC50 (M) value of about 5.5, even under conditions where PKA activity was either abolished by both the PKA antagonists KT5720 and protein kinase inhibitor fragment 6–22 amide or enhanced by the PKA stimulators 6-Bnz-cAMP and 8-Br-cAMP. Inhibition of ICa1.2 by H-89 appeared almost irreversible upon washout, was charge carrier- and voltage-dependent, and antagonised by the Cav1.2 channel agonist (S)-(-)-Bay K 8644. H-89 did not alter both potency and efficacy of verapamil, did not affect current kinetics or voltage-dependent activation, while shifting to the left the 50% voltage of inactivation in a concentration-dependent manner. H-89 docked at the α1C subunit in a pocket region close to that of (S)-(-)-Bay K 8644 docking, forming a hydrogen bond with the same, key amino acid residue Tyr-1489. Finally, both high K+- and (S)-(-)-Bay K 8644-induced contractions of rings were fully reverted by H-89. In conclusion, these results indicate that H-89 inhibited vascular ICa1.2 and, consequently, the contractile function through a PKA-independent mechanism. Therefore, caution is recommended when interpreting experiments where H-89 is used to inhibit vascular smooth muscle PKA.

Functional, electrophysiological and molecular docking analysis of the modulation of Cav 1.2 channels in rat vascular myocytes by murrayafoline A.

The carbazole alkaloid murrayafoline A (MuA) enhances contractility and the Ca(2+) currents carried by the Cav 1.2 channels [ICa1.2 ] of rat cardiomyocytes. As only few drugs stimulate ICa1.2 , this study was designed to analyse the effects of MuA on vascular Cav 1.2 channels. Vascular activity was assessed on rat aorta rings mounted in organ baths. Cav 1.2 Ba(2+) current [IBa1.2 ] was recorded in single rat aorta and tail artery myocytes by the patch-clamp technique. Docking at a 3D model of the rat, α1c central pore subunit of the Cav 1.2 channel was simulated in silico. In rat aorta rings MuA, at concentrations ≤14.2 μM, increased 30 mM K(+) -induced tone and shifted the concentration-response curve to K(+) to the left. Conversely, at concentrations >14.2 μM, it relaxed high K(+) depolarized rings and antagonized Bay K 8644-induced contraction. In single myocytes, MuA stimulated IBa1.2 in a concentration-dependent, bell-shaped manner; stimulation was stable, incompletely reversible upon drug washout and accompanied by a leftward shift of the voltage-dependent activation curve. MuA docked at the α1C subunit central pore differently from nifedipine and Bay K 8644, although apparently interacting with the same amino acids of the pocket. Neither Bay K 8644-induced stimulation nor nifedipine-induced block of IBa1.2 was modified by MuA. Murrayafoline A is a naturally occurring vasoactive agent able to modulate Cav 1.2 channels and dock at the α1C subunit central pore in a manner that differed from that of dihydropyridines.

Magnolol inhibits colonic motility through down-regulation of voltage-sensitive L-type Ca2+ channels of colonic smooth muscle cells in rats

This study aimed to investigate the effect of magnolol (5,5′-diallyl-2,2′-biphenyldiol) on contraction in distal colonic segments of rats and the underlying mechanisms. Colonic segments were mounted in organ baths for isometric force measurement. Whole-cell voltage-sensitive L-type Ca2+ currents were recorded on isolated single colonic smooth muscle cells using patch-clamp technique. The spontaneous contractions and acetylcholine (ACh)- and Bay K 8644-induced contractions were inhibited by magnolol (3–100μM). In the presence of Bay K8644 (100nM), magnolol (10–100μM) inhibited the contraction induced by 10μM ACh. By contrast, tetrodotoxin (100nM) and Nώ-nitro-l-arginine methyl ester (l-NAME 100μM) did not change the inhibitory effect of magnolol (10μM). In addition, magnolol (3–100μM) inhibited the L-type Ca2+ currents. The present results suggest that magnolol inhibits colonic smooth muscle contraction through downregulating L-type Ca2+ channel activity.

NS-398 (a Selective Cyclooxygenase-2 Inhibitor) Decreases Agonist-Induced Contraction of the Human UreterviaCalcium Channel Inhibition

Recently, it has been demonstrated that ureteral obstruction is associated with increased cyclooxygenase (COX)-2 expression and that selective COX-2 inhibitors provide potent analgesia with fewer side effects in patients with ureteral stones. Moreover, selective COX-2 inhibitors have been shown to decrease in vitro contractility of the human ureter. We aimed at evaluating the effects of the selective COX-2 inhibitor NS-398 on human ureteral smooth muscle contractility and compare its potency with that of nonselective COX inhibitors, COX-1 inhibitors, and other COX-2 inhibitors. Ureteral samples were obtained from human adult subjects undergoing radical nephrectomy. After isolating the upper ureteral strips, we analyzed the contractile responses of the ureteral strips to high potassium (KCl 35 mM) and Bay K 8644 and the relaxation responses of a nonspecific COX inhibitor (indomethacin), a COX-1 inhibitor (SC-560), and a COX-2 inhibitor (NS-398 and celecoxib) to KCl and Bay K 8644-induced contraction by measuring isometric tension. NS-398 produced dose-dependent (10⁻⁹-10⁻⁵ M) relaxation of KCl (35 mM)-precontracted strips of the ureter, whereas indomethacin (10⁻⁸-10⁻⁵ M) and SC-560 (10⁻⁹-10⁻⁵ M) did not. Both tonic and phasic contraction of Bay K 8644 (methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-2(trifluoromethylphenyl)pyridine-5-carboxylate) (1 μM) were significantly inhibited by NS-398 (10⁻⁵ M). Another selective COX-2 inhibitor, celecoxib, did not show potent inhibitory effects as strong as those of NS-398. We concluded that NS-398 reduces tonic or phasic contraction by inhibiting the action of voltage-dependent calcium channels. NS-398 has dual inhibitory effects with COX-2 inhibition on ureteral spasms due to renal or ureteral colic.

Failure of Bay K 8644 to induce RhoA kinase-dependent calcium sensitization in rabbit blood vessels.

RhoA kinase (ROCK) participates in K(+) depolarization (KCl)-induced Ca(2+) sensitization of contraction. Whether constitutive, depolarization- or Ca(2+)-activated ROCK plays the major role in this signalling system remains to be determined. Here, we determined whether Bay K 8644, a dihydropyridine that promotes Ca(2+) channel clusters to operate in a persistent Ca(2+) influx mode, could cause ROCK-dependent Ca(2+) sensitization. Renal and femoral artery rings from New Zealand white rabbits were contracted with Bay K 8644. Tissues were frozen and processed to measure active RhoA and ROCK substrate (myosin phosphatase targeting subunit, MYPT1) and myosin light chain (MLC) phosphorylation, or loaded with fura-2 to measure intracellular free Ca(2+) ([Ca(2+)](i)). Effects of selective inhibitors of contraction were assessed in resting (basal) tissues and those contracted with Bay K 8644. Bay K 8644 produced strong increases in [Ca(2+)](i), MLC phosphorylation and tension, but not in MYPT1 phosphorylation. ROCK inhibition by H-1152 abolished basal MYPT1-pT853, diminished basal MLC phosphorylation and inhibited Bay K 8644-induced increases in MLC phosphorylation and tension. MLC kinase inhibition by wortmannin abolished Bay K 8644-induced contraction and increase in MLC phosphorylation but did not inhibit basal MYPT1-pT853. H-1152 and wortmannin had no effect on MYPT1-pT696, but 1 microM staurosporine inhibited basal MYPT1-pT853, MYPT1-pT696 and MLC phosphorylation. These data suggest that the constitutive activities of ROCK and a staurosporine-sensitive kinase regulate basal phosphorylation of MYPT1, which participates along with activation of MLC kinase in determining the strength of contraction induced by the Ca(2+) agonist, Bay K 8644.

Enhancement of methoxamine‐induced contractile responses of rat ventricular muscle in streptozotocin‐induced diabetes is associated with α1A adrenoceptor upregulation

To clarify the time-related changes in cardiac function and the mechanism underlying the cardiac dysfunction present in diabetes mellitus, we studied mechanical responses induced by alpha(1)- and beta-adrenoceptors, the Ca(2+)-entry promoter Bay K 8644- and ryanodine (an agent known to inhibit Ca(2+) release from the sarcoplasmic reticulum) in papillary muscles from streptozotocin (STZ)-induced diabetic and age-matched control rats. Male Wistar rats received a single injection of STZ (60 mg kg(-1)) via the tail vein to induce diabetes. For the mechanical studies, papillary muscle preparations were suspended in an organ bath and isometric contractions were measured in 1-, 4-, and 10-week STZ-induced diabetic and age-matched control rats. In 1-week diabetic rats, the contractions induced by isoproterenol, methoxamine and Bay K 8644 were unchanged (vs. age-matched controls). In 4-week diabetic rats, (a) the isoproterenol- and Bay K 8644-induced contractions were impaired, (b) sensitivity to ryanodine was reduced, whereas (c) the methoxamine-induced contraction was unchanged. In 10-week diabetic rats, the isoproterenol- and Bay K 8644-induced contractile responses were impaired and the sensitivity to ryanodine was reduced, but in sharp contrast the methoxamine-induced contraction was enhanced. Both the mRNA level for the alpha(1A) adrenoceptor (but not the alpha(1B) or alpha(1D) mRNAs) and alpha(1A) adrenoceptor protein were increased in 10-week diabetic rats (vs. age-matched controls). These results suggest that impairments of beta-adrenergic and Ca(2+)-handling mechanisms occur early in the development of cardiomyopathy in STZ-induced diabetic rats, and that this is followed by augmentation of alpha(1A) adrenoceptor-mediated inotropy due to alpha(1A) adrenoceptor upregulation.

(-)-Epigallocatechin-3-gallate induces contraction of the rat aorta by a calcium influx-dependent mechanism.

Although the consumption of tea has been associated with beneficial cardiovascular effects, (-)-epigallocatechin-3-gallate (EGCG), the most abundant catechin in this beverage has shown seemingly contradictory actions on vascular tissues, for example vasorelaxant activity that could contribute favourably to prevention of cardiovascular disease, and contractile activity that could act in the opposite direction. The purpose of the present work was to study the contractile effects of EGCG on isolated rat thoracic aorta rings and its effects on the cytosolic free [Ca(2+)] ([Ca(2+)](i)) measured with fura-2 in cultured rat aortic smooth muscle cell line. In partially depolarised (15 mM KCl) aortic rings EGCG (30-300 microM), (+/-)-BAY K 8644 (0.1 microM) and thapsigargin (1 microM) induced a Ca(2+)-dependent, endothelium-independent contraction associated with [Ca(2+)](i) elevation in RASMC. EGCG enhanced the responses elicited by (+/-)-BAY K 8644 and thapsigargin both in aortic rings and in RASMC. Nifedipine totally inhibited the (+/-)-BAY K 8644-induced contraction, but only partially blocked the contractile responses to EGCG and thapsigargin, while SKF 96365 abolished both responses. The effects of these channel blockers were associated with a decrease in [Ca(2+)](i) in RASMC. Re-introduction of Ca(2+) in the medium after depletion of intracellular Ca(2+) stores with thapsigargin in a Ca(2+)-free solution elicited a contraction of aortic rings and an increase in [Ca(2+)](i) in RASMC. In both cases, this response was partially sensitive to nifedipine, abolished by SKF 96365 and clearly enhanced by EGCG. These results suggest that EGCG induces a transient endothelium-independent contraction in the rat aorta, probably by increasing smooth vascular cell membrane permeability to Ca(2+) through both non-specific and dihydropyridine-sensitive Ca(2+) channels.

A mechanism of vasodilatory action of polyamines and acetylpolyamines: possible involvement of their Ca2+ antagonistic properties.

Polyamines, a class of low-molecular weight organic polycations, have been shown to produce relaxing effects in vascular smooth muscles, although the mechanism has not been carefully examined. In this study, the mechanism of vascular action of polyamines and their metabolites, acetylpolyamines, was pharmacologically examined in the rabbit isolated thoracic aorta focusing on an endothelium-dependent component of vasodilatation and Ca2+ influx through plasma membrane channels. Both polyamines and acetylpolyamines (except N1-acetylputrescine, which produced no response or very slight contraction) caused concentration-dependent relaxation in preconstricted aortic rings containing an intact endothelium. Aortic rings denuded of endothelium were also responsive to both polyamines and acetylpolyamines. Inhibitors of nitric oxide (reduced haemoglobin and Nomega-nitro-L-arginine methyl ester), vasodilator prostaglandins (indomethacin) and guanylyl cyclase (methylene blue) did not affect the relaxation induced by both polyamines and acetylpolyamines in either endothelium-intact or -denuded aortic rings. Both polyamines and acetylpolyamines inhibited the concentration-dependent contraction for phenylephrine and K+. The Ca2+ agonist Bay K 8644 induced concentration-dependent contraction in segments of rabbit aorta partially depolarized with 15 mM KCl, and both polyamines and acetylpolyamines relaxed the Bay K 8644-induced contraction in a concentration-dependent manner. Interestingly, both polyamines and acetylpolyamines also decreased contractions evoked by the Ca2+ ionophore A23187. The concentration-response curve to exogenous Ca2+ in K+-depolarization medium (K+ = 120 mM) was shifted to the right by both polyamines and acetylpolyamines. The response elicited by Ca2+ was increased by Bay K 8644 (10(-6) M), and this potentiation was also inhibited by both polyamines and acetylpolyamines. The results indicate that both polyamines and acetylpolyamines can induce vasorelaxation of rabbit thoracic aorta by an endothelium-independent mechanism in-vitro and relax vascular smooth muscle by acting at the plasma membrane level, decreasing the influx of Ca2+. Therefore, polyamines and acetylpolyamines may have Ca2+ antagonistic properties which may, in part, be involved in the mechanism of rabbit aortic vascular smooth muscle relaxation.

In vitro and in vivo vascular responses to the L-type calcium channel activator, Bay K 8644, in rats with cirrhosis

A substance which increases the entry of extracellular calcium into arterial smooth muscle may decrease cirrhosis-induced vasodilation. The aim of the present study was to measure the effects of the L-type Ca2+ channel activator, Bay K 8644, on the haemodynamics of rats with cirrhosis. Vascular reactivity to this substance was also investigated. Splanchnic and systemic haemodynamic responses to Bay K 8644 (50 microg/kg) were measured in cirrhotic and normal rats. Contraction induced by 0.1 micromol/L Bay K 8644 was measured in arterial rings (aorta and superior mesenteric artery) from cirrhotic and normal rats. In cirrhotic rats, Bay K 8644 significantly decreased portal pressure (15%) and portal tributary blood flow (24%), significantly increased portal territory vascular resistance (54%) and did not significantly change hepatocollateral vascular resistance. Bay K 8644 significantly increased arterial pressure (7%) and systemic vascular resistance (24%) and did not change the cardiac index. In normal rats, Bay K 8644 significantly increased vascular resistance (150%) in portal, hepatocollateral and systemic territories and significantly decreased the cardiac index (44%). Changes in portal territory, hepatocollateral and systemic vascular resistances were significantly less marked in cirrhotic than in normal rats. In rings from the aorta and superior mesenteric artery, Bay K 8644-induced contraction was significantly lower in cirrhotic than in normal rats. In conclusion, in rats with cirrhosis, Bay K 8644 administration reduced vasodilation in splanchnic and systemic arteries and did not affect hepatocollateral vascular resistance. The Bay K 8644-induced reduction in splanchnic vasodilation caused a decrease in portal hypertension. This study also shows that Bay K 8644-induced vascular contraction was less marked in cirrhotic than in normal rats, in systemic and splanchnic vascular beds.

Effects of endothelins, Bay K 8644 and other oxytocics in non-pregnant and late pregnant rat isolated uterus

Endothelins 1, 2 and 3 (ET-1, ET-2 and ET-3; 1–30 nM) caused long-lasting concentration-dependent tonic contractions of uterine strips from non-pregnant rats. The potency of ET-1 (EC 50 7 nM) was similar to that of angiotensin II (AII) and greater than that of ET-2 or ET-3 (EC 50s ⩾ 10 nM), bradykinin, Bay K 8644, oxytocin (OT), 5-hydroxytryptamine, prostaglandin F 2α (PGF 2α) or acetylcholine. Strips from 21-day pregnant rats were 2- to 3-fold more sensitive to ET-1, AII, OT and PGF 2α and 200-fold more sensitive to Bay K 8644 than non-pregnant preparations. The development of tonic responses to ET-1 (30 nM) and of phasic-rhythmic ones to Bay K 8644 (300 nM) was fully prevented in strips from non-pregnant rats bathed in Ca 2+-free medium, but stepwise reintroduction of Ca 2+ (0.03–3 mM) to the solution allowed the manifestation of contractions in response to both agonists. Responses to ET-1 required less Ca 2+ than those to Bay K 8644. Strips challenged with ET-1 while in Ca 2+-free medium developed greater contractions upon reintroduction of Ca 2+ than preparations stimulated with the peptide in normal medium. The reverse occured with Bay K 8644-induced contractions. Nicardipine (10 nM) abolished the responses of strips from non-pregnant rats to Bay K 8644 (300 nM), but only attenuated ET-1-induced (30 nM) contraction. A subthreshold concentration of ET-1 (0.3 nM) augmented the responses to PGF 2α or 5-HT and to AII in preparations taken from non-pregnant and pregnant rats. Thus, ET-1 is one of the most powerful known constrictors of the rat isolated uterus. Its action is enhanced during late pregnancy and is mediated by a mechanism distinct from that of Bay K 8644. It is suggested that ETs may play significant roles in the control of uterine contractility.

Evidence for the mechanism of the inhibitory action of jatrophone in the isolated rat uterine muscle

1. 1. Jatrophone (JAT), a diterpene isolated from the plant Jatropha elliptica (1–300 μM), caused a concentration-dependent relaxation effect against acetylcholine (Ach)-oxytocin (Ot)- and KCl-induced uterine sustained contraction. The relative potency order was: Ach > Ot > KCl. 2. 2. The relaxant effect of JAT was not modified by phorbol ester, forskolin, MIX, TMB-8 and W-7. The increase concentration of calcium (0.2–2 mM) in the medium did not reverse the inhibitory effect caused by JAT. 3. 3. Pre-incubation of the preparations with JAT (16–32 μM) for 20 min, caused a concentration-dependent inhibition of KCl-induced contractile response. At 30 μM, JAT inhibited in an apparently non-competitive manner CaCl 2-induced contraction in K +-depolarized preparations. High concentrations of JAT (100 μM) also caused a time-dependent relaxation in CaCl 2-induced sustained uterine contraction ( T 1 2 = approx. 15 min ). 4. 4. JAT (30 μM) inhibited the dihydropyridine calcium channel agonist Bay K 8644-induced uterine contraction in an apparently non-competitive fashion, while verapamil (0.1 μM) caused in rightward displacement of Bay K 8644 contraction and marked inhibition of the maximal response.

Paradoxical augmentation of (−)Bay K 8644-induced calcium influx by nitrendipine

(−)Bay K 8644 produced a concentration-dependent contraction of porcine coronary artery rings with the maximal contraction at 10 −6 M. Pretreatment of the rings with 10 −6 M nitrendipine inhibited (−)Bay K 8644-induced contraction, while pretreatment with 10 −8 M nitrendipine potentiated the contraction elicited by (−)Bay K 8644. (−)Bay K 8644 (10 −6M) significantly stimulated Ca 2+ influx. Although 10 −8 M nitrendipine never stimulated Ca 2+ influx, Ca 2+ influx induced by (−)Bay K 8644 was significantly potentiated by pretreatment with 10 −8 M nitrendipine. Pretreatment with 10 −6 M nitrendipine significantly decreased Ca 2+ influx in tissues treated with (−)Bay K 8644. Our results suggest that the increased Ca 2+ influx might be involved in the mechanisms by which (−)Bay K 8644-induced contraction was potentiated by pretreatment with nitrendipine.

Comparison of the Effects of the Vasoconstrictive Dihydropyridines BAY K 8644 and CGP 28392 on Isolated Rat Aorta

The effect of BAY K 8644 was compared to that of CGP 28392 in isolated rat thoracic aorta. In low concentration both dihydropyridines had a direct concentration dependent contractile effect that could be eliminated in Ca-free medium and by the Ca-antagonist nifedipine. Readdition of Ca to the Ca-free medium restored the contractile response to both agents. In high concentrations (greater than or equal to 10(-5) M), both agents depressed active tone. In physiological salt solution containing 15.8 mM, potassium BAY K 8644 was far more potent in contracting rat aorta than CGP 28392. The responses to BAY K 8644 and CGP 28392 were not affected by alpha- and beta-adrenoceptorblockade. Both compounds shifted the noradrenaline- and potassium-concentration response curve to the left and increased the maximal response. Ultraviolet radiation (UVR) had a reversible and marked effect on BAY K 8644 and no effect on CGP 28392 contractions. The UVR-mediated relaxation of BAY K 8644-induced contractions could partly be eliminated by raising extracellular Ca. The results showed that differences according to potency and mode of action exist between BAY K 8644 and CGP 28392 in rat aorta. To explain the effect of UVR on BAY K 8644-induced contractions, a modification of a Ca channel model is proposed that implies the existence of "photosensitive proteins" located to "Ca antagonist" binding sites. According to this hypothesis, UVR may induce a reversible binding of BAY K 8644 to "Ca antagonist receptors."

Nitrendipine potentiates bay K 8644-induced contraction of isolated porcine coronary artery: Evidence for functionally distinct dihydropyridine receptor subtypes

Bay k 8644 and nitrendipine, dihydropyridines classified as calcium channel agonist and antagonist, respectively, produced concentration-dependent biphasic responses (contraction and relaxation) in porcine coronary artery rings. Nitrendipine relaxed rings (IC 50 = 60 nM) that were contracted with 100 nM Bay k 8644. Pretreatment of rings with 60 nM nitrendipine caused paradoxical potentiation of Bay k 8644-induced contraction. The data are consistent with a model that consists of two functionally-distinct dihydropyridine “receptors” with which Bay k 8644 and nitrendipine interact as partial agonists. We propose that these excitatory and inhibitory dihydropyridine receptor subtypes mediate contraction and relaxation, respectively, by dihydropyridines.

Effects of a novel calcium channel agonist dihydropyridine analogue, Bay k 8644, on pig coronary artery: biphasic mechanical response and paradoxical potentiation of contraction by diltiazem and nimodipine.

Bay k 8644 is a structural analogue of the 1,4-dihydropyridines whose pharmacological actions on heart and vascular smooth muscle are opposite from those of nifedipine and other similar calcium antagonists. We have examined the action of Bay k 8644 ("calcium channel agonist") on isolated porcine coronary artery rings. The interactions between Bay k 8644 and the vasodilators isosorbide dinitrate (ISDN), diltiazem, and nimodipine were quantitated. Bay k 8644 produced a biphasic, dose-dependent mechanical response, with contraction occurring over the concentration range of 1-350 nM (ED50 = 11.4 nM) and relaxation observed at concentrations greater than 350 nM (IC50 = 5.7 microM). ISDN, diltiazem, and nimodipine relaxed, in a dose-dependent manner, maximal Bay k 8644-induced contractions. When the coronary rings were pretreated for 25-90 min with 80% inhibitory concentrations of these vasodilators, there was little or no effect by ISDN on Bay k 8644-induced contractions; however, there was a surprising potentiation by diltiazem and by nimodipine. Pretreatment of coronary rings with higher concentrations of ISDN or diltiazem caused an inhibition of Bay k 8644-induced contraction, while pretreatment with higher concentrations of nimodipine caused further potentiation of contraction elicited by Bay k 8644. Bay k 8644 increased the tension developed in response to high potassium (potential-operated channel activation) or histamine (receptor-operated channel activation). To account for the biphasic response to Bay k 8644 (dose-dependent contraction and relaxation), and the unexpected potentiation of Bay k 8644-induced contraction by nimodipine and by diltiazem, a molecular model is proposed for vascular smooth muscle in which Bay k 8644 functions as a partial calcium channel agonist at two functionally distinct 1,4-dihydropyridine "receptor sites."