Cyclic nucleotides regulate the activity of L-type calcium channels in smooth muscle cells from rat portal vein

Oxidative Regulation of Vascular Cav1.2 Channels Triggers Vascular Dysfunction in Hypertension-Related Disorders

Blood pressure is determined by cardiac output and peripheral vascular resistance. The L-type voltage-gated Ca²⁺ (Ca_v1.2) channel in small arteries and arterioles plays an essential role in regulating Ca²⁺ influx, vascular resistance, and blood pressure. Hypertension and preeclampsia are characterized by high blood pressure. In addition, diabetes has a high prevalence of hypertension. The etiology of these disorders remains elusive, involving the complex interplay of environmental and genetic factors. Common to these disorders are oxidative stress and vascular dysfunction. Reactive oxygen species (ROS) derived from NADPH oxidases (NOXs) and mitochondria are primary sources of vascular oxidative stress, whereas dysfunction of the Ca_v1.2 channel confers increased vascular resistance in hypertension. This review will discuss the importance of ROS derived from NOXs and mitochondria in regulating vascular Ca_v1.2 and potential roles of ROS-mediated Ca_v1.2 dysfunction in aberrant vascular function in hypertension, diabetes, and preeclampsia.

Vasorelaxant effects of 2-nitro-1-phenyl-1-propanol in rat aorta

2-Nitro-1-phenyl-1-propanol (NPP) is a nitro alcohol that is known as an intermediate in the synthesis of sympathomimetic agents, such as norephedrine. The present study investigated the vasoactive effects of NPP on rat aorta. In endothelium-intact aortic rings, NPP fully relaxed contractions that were induced by phenylephrine, KCl, and U-46619. The relaxant effects of NPP on phenylephrine-elicited contractions remained unaffected by NG-nitro-l-arginine methyl ester (l-NAME), indomethacin, propranolol, tetraethylammonium, 4-aminopyridine, and glibenclamide. Conversely, pretreatment with 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), cis-N-(2-phenylcyclopentyl)-azacyclotridec-1-en-2-amine hydrochloride (MDL-12,330A), and N-[2-(P-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride (H-89) reduced the ability of NPP to relax contractions that were elicited by phenylephrine. NPP inhibited the vasoconstrictor response that was induced by Ca²⁺ in aortic rings that were stimulated by pharmacomechanical or electromechanical coupling with phenylephrine and 60 mmol/L KCl, respectively, and after the depletion of intracellular Ca²⁺ stores. Such effects of NPP were significantly reversed by pretreatment with the guanylyl cyclase inhibitor ODQ and weakly influenced by the adenylyl cyclase inhibitor MDL-12,330A. In Ca²⁺ -free medium, NPP inhibited transient contractions that were induced by phenylephrine but not caffeine. In homogenates of aortic rings, NPP increased cyclic guanosine 3',5'-monophosphate (cGMP) and cyclic adenosine 3'-5'-monophosphate levels, but this effect was statistically significant only for cGMP. In conclusion, in contrast to the vasoconstrictor amine norephedrine, NPP is a vasodilator in rat aorta, and its relaxant effects are likely attributable to cGMP production.

Excitation-Contraction Coupling and Excitation-Transcription Coupling in Blood Vessels: Their Possible Interactions in Hypertensive Vascular Remodeling

Vascular smooth muscle cells (VSMC) display considerable phenotype plasticity which can be studied in vivo on vascular remodeling which occurs during acute or chronic vascular injury. In differentiated cells, which represent contractile phenotype, there are characteristic rapid transient changes of intracellular Ca2+ concentration ([Ca2+]i), while the resting cytosolic [Ca2+]i concentration is low. It is mainly caused by two components of the Ca2+ signaling pathways: Ca2+ entry via L-type voltage-dependent Ca2+ channels and dynamic involvement of intracellular stores. Proliferative VSMC phenotype is characterized by long-lasting [Ca2+]i oscillations accompanied by sustained elevation of basal [Ca2+]i. During the switch from contractile to proliferative phenotype there is a general transition from voltage-dependent Ca2+ entry to voltage-independent Ca2+ entry into the cell. These changes are due to the altered gene expression which is dependent on specific transcription factors activated by various stimuli. It is an open question whether abnormal VSMC phenotype reported in rats with genetic hypertension (such as spontaneously hypertensive rats) might be partially caused by a shift from contractile to proliferative VSMC phenotype.

Nifedipine-Sensitive Blood Pressure Component in Hypertensive Models Characterized by High Activity of Either Sympathetic Nervous System or Renin-Angiotensin System

High blood pressure (BP) of spontaneously hypertensive rats (SHR) is maintained by enhanced activity of sympathetic nervous system (SNS), whereas that of Ren-2 transgenic rats (Ren-2 TGR) by increased activity of renin-angiotensin system (RAS). However, both types of hypertension are effectively attenuated by chronic blockade of L-type voltage-dependent calcium channel (L-VDCC). The aim of our study was to evaluate whether the magnitude of BP response elicited by acute nifedipine administration is proportional to the alterations of particular vasoactive systems (SNS, RAS, NO) known to modulate L-VDCC activity. We therefore studied these relationships not only in SHR, in which mean arterial pressure was modified in a wide range of 100-210 mm Hg by chronic antihypertensive treatment (captopril or hydralazine) or its withdrawal, but also in rats with augmented RAS activity such as homozygous Ren-2 TGR, pertussis toxin-treated SHR or L-NAME-treated SHR. In all studied groups the magnitude of BP response to nifedipine was proportional to actual BP level and it closely correlated with BP changes induced by acute combined blockade of RAS and SNS. BP response to nifedipine is also closely related to the degree of relative NO deficiency. This was true for both SNS- and RAS-dependent forms of genetic hypertension, suggesting common mechanisms responsible for enhanced L-VDCC opening and/or their upregulation in hypertensive animals. In conclusions, BP response to nifedipine is proportional to the vasoconstrictor activity exerted by both SNS and RAS, indicating a key importance of these two pressor systems for actual L-VDCC opening necessary for BP maintenance.

Involvement of BKCa and KV potassium channels in cAMP-induced vasodilatation: their insufficient function in genetic hypertension

Spontaneously hypertensive rats (SHR) are characterized by enhanced sympathetic vasoconstriction, whereas their vasodilator mechanisms are relatively attenuated compared to their high BP. The objective of our in vivo study was to evaluate whether the impaired function of BKCa and/or KV channels is responsible for abnormal cAMP-induced vasodilatation in genetic hypertension. Using conscious SHR and normotensive WKY rats we have shown that under the basal conditions cAMP overproduction elicited by the infusion of beta-adrenoceptor agonist (isoprenaline) caused a more pronounced decrease of baseline blood pressure (BP) in SHR compared to WKY rats. Isoprenaline infusion prevented BP rises induced by acute NO synthase blockade in both strains and it also completely abolished the fully developed BP response to NO synthase blockade. These cAMP-induced vasodilator effects were diminished by the inhibition of either BKCa or KV channels in SHR but simultaneous blockade of both K(+) channel types was necessary in WKY rats. Under basal conditions, the vasodilator action of both K(+) channels was enhanced in SHR compared to WKY rats. However, the overall contribution of K(+) channels to cAMP-induced vasodilator mechanisms is insufficient in genetic hypertension since a concurrent activation of both K(+) channels by cAMP overproduction is necessary for the prevention of BP rise elicited by acute NO/cGMP deficiency in SHR. This might be caused by less effective activation of these K(+) channels by cAMP in SHR. In conclusion, K(+) channels seem to have higher activity in SHR, but their vasodilator action cannot match sufficiently the augmented vasoconstriction in this hypertensive strain.

Pathophysiology of hypertension in the absence of nitric oxide/cyclic GMP signaling

The nitric oxide (NO)-cyclic guanosine monophosphate (cGMP) signaling system is a well-characterized modulator of cardiovascular function, in general, and blood pressure, in particular. The availability of mice mutant for key enzymes in the NO-cGMP signaling system facilitated the identification of interactions with other blood pressure modifying pathways (e.g. the renin-angiotensin-aldosterone system) and of gender-specific effects of impaired NO-cGMP signaling. In addition, recent genome-wide association studies identified blood pressure-modifying genetic variants in genes that modulate NO and cGMP levels. Together, these findings have advanced our understanding of how NO-cGMP signaling regulates blood pressure. In this review, we will summarize the results obtained in mice with disrupted NO-cGMP signaling and highlight the relevance of this pathway as a potential therapeutic target for the treatment of hypertension.

Cyclic nucleotide-dependent relaxation pathways in vascular smooth muscle

Vascular smooth muscle tone is controlled by a balance between the cellular signaling pathways that mediate the generation of force (vasoconstriction) and release of force (vasodilation). The initiation of force is associated with increases in intracellular calcium concentrations, activation of myosin light-chain kinase, increases in the phosphorylation of the regulatory myosin light chains, and actin-myosin crossbridge cycling. There are, however, several signaling pathways modulating Ca(2+) mobilization and Ca(2+) sensitivity of the contractile machinery that secondarily regulate the contractile response of vascular smooth muscle to receptor agonists. Among these regulatory mechanisms involved in the physiological regulation of vascular tone are the cyclic nucleotides (cAMP and cGMP), which are considered the main messengers that mediate vasodilation under physiological conditions. At least four distinct mechanisms are currently thought to be involved in the vasodilator effect of cyclic nucleotides and their dependent protein kinases: (1) the decrease in cytosolic calcium concentration ([Ca(2+)]c), (2) the hyperpolarization of the smooth muscle cell membrane potential, (3) the reduction in the sensitivity of the contractile machinery by decreasing the [Ca(2+)]c sensitivity of myosin light-chain phosphorylation, and (4) the reduction in the sensitivity of the contractile machinery by uncoupling contraction from myosin light-chain phosphorylation. This review focuses on each of these mechanisms involved in cyclic nucleotide-dependent relaxation of vascular smooth muscle under physiological conditions.

cGMP reduces the sarcoplasmic reticulum Ca2+ loading in airway smooth muscle cells: a putative mechanism in the regulation of Ca2+ by cGMP

Ca(2+) and cGMP have opposite roles in many physiological processes likely due to a complex negative feedback regulation between them. Examples of opposite functions induced by Ca(2+) and cGMP are smooth muscle contraction and relaxation, respectively. A main Ca(2+) storage involved in contraction is sarcoplasmic reticulum (SR); nevertheless, the role of cGMP in the regulation of SR-Ca(2+) has not been completely understood. To evaluate this role, intracellular Ca(2+) concentration ([Ca(2+)]i) was determinated by a ratiometric method in isolated myocytes from bovine trachea incubated with Fura-2/AM. The release of Ca(2+) from SR induced by caffeine was transient, whereas caffeine withdrawal was followed by a [Ca(2+)]i undershoot. Caffeine-induced Ca(2+) transient peak and [Ca(2+)]i undershoot after caffeine were reproducible in the same cell. Dibutyryl cGMP (db-cGMP) blocked the [Ca(2+)]i undershoot and reduced the subsequent caffeine peak (SR-Ca(2+) loading). Both, the opening of SR channels with ryanodine (10 μM) and the blockade of SR-Ca(2+) ATPase with cyclopiazonic acid inhibited the [Ca(2+)]i undershoot as well as the SR-Ca(2+) loading. The addition of db-cGMP to ryanodine (10 μM) incubated cells partially restored the SR-Ca(2+) loading. Cyclic GMP enhanced [Ca(2+)]i undershoot induced by the blockade of ryanodine channels with 50 μM ryanodine. In conclusion, the reduction of SR-Ca(2+) content in airway smooth muscle induced by cGMP can be explained by the combination of SR-Ca(2+) loading and the simultaneous release of SR-Ca(2+). The reduction of SR-Ca(2+) content induced by cGMP might be a putative mechanism limiting releasable Ca(2+) in response to a particular stimulus.

Altered neural and vascular mechanisms in hypertension

Essential hypertension is a multifactorial disorder which belongs to the main risk factors responsible for renal and cardiovascular complications. This review is focused on the experimental research of neural and vascular mechanisms involved in the high blood pressure control. The attention is paid to the abnormalities in the regulation of sympathetic nervous system activity and adrenoceptor alterations as well as the changes of membrane and intracellular processes in the vascular smooth muscle cells of spontaneously hypertensive rats. These abnormalities lead to increased vascular tone arising from altered regulation of calcium influx through L-VDCC channels, which has a crucial role for excitation-contraction coupling, as well as for so-called "calcium sensitization" mediated by the RhoA/Rho-kinase pathway. Regulation of both pathways is dependent on the complex interplay of various vasodilator and vasoconstrictor stimuli. Two major antagonistic players in the regulation of blood pressure, i.e. sympathetic nervous system (by stimulation of adrenoceptors coupled to stimulatory and inhibitory G proteins) and nitric oxide (by cGMP signaling pathway), elicit their actions via the control of calcium influx through L-VDCC. However, L-type calcium current can also be regulated by the changes in membrane potential elicited by the activation of potassium channels, the impaired function of which was detected in hypertensive animals. The dominant role of enhanced calcium influx in the pathogenesis of high blood pressure of genetically hypertensive animals is confirmed not only by therapeutic efficacy of calcium antagonists but especially by the absence of hypertension in animals in which L-type calcium current was diminished by pertussis toxin-induced inactivation of inhibitory G proteins. Although there is considerable information on the complex neural and vascular alterations in rats with established hypertension, the detailed description of their appearance during the induction of hypertension is still missing.

Hyperpolarization-activated ion channels as targets for nitric oxide signalling in deep cerebellar nuclei

Most biological effects of nitric oxide (NO) in the brain are mediated by guanylyl cyclase-coupled NO receptors, whose activation results in increased intracellular cGMP levels. Apart from protein kinase activation little is known about subsequent cGMP signal transduction. In optic nerve axons, hyperpolarization-activated cyclic nucleotide-modulated cation (HCN) channels, which bind cGMP or cAMP directly, were recently suggested to be a target. The aim here was to test this possibility more directly. Neurones of the rat deep cerebellar nuclei were selected for this purpose, their suitability being attested by immunocytochemistry showing that the principal neurones expressed guanylyl cyclase protein and that NO synthase-containing fibres were abundant in the neuropil. Using whole-cell voltage-clamp recording, HCN channels in the neurones were activated in response to isoprenaline and exogenous cAMP but only occasionally did they respond to NO, although exogenous cGMP was routinely effective. With the less invasive sharp microelectrode recording technique, however, exogenous NO modulated the channels reproducibly, as measured by the size of the HCN channel-mediated voltage sag following hyperpolarization. Moreover, NO also blunted the subsequent rebound depolarizing potentials, consistent with it increasing the hyperpolarization-activated current. Optimizing the whole-cell solution to improve the functioning of NO-activated guanylyl cyclase failed to restore NO sensitivity. Minimizing cellular dialysis by using the perforated-patch technique, however, was successful. The results provide evidence that HCN channels are potential downstream mediators of NO signalling in deep cerebellar nuclei neurones and suggest that the more general importance of this transduction pathway may have been overlooked previously because of unsuitable recording methods.

Sodium ferulate attenuates anoxia/reoxygenation-induced calcium overload in neonatal rat cardiomyocytes by NO/cGMP/PKG pathway

Development of intracellular calcium overload is an important pathophysiological factor in myocardial ischemia/reperfusion or anoxia/reoxygenation injury. Recent studies have shown that Sodium Ferulate (SF) stimulates nitric oxide (NO) production and exerts a cardioprotective effect in the ischemia-reperfused heart. However, it has not been determined whether the cardioprotection of SF is associated with suppression of Ca(2+) overload via NO/cyclic GMP (cGMP)/cGMP-dependent protein kinase (PKG) pathway. In this work, after cardiomyocytes were incubated with 100, 200, 400, or 800 microM SF for 3 h, anoxia/reoxygenation injury was induced and intracellular Ca(2+) concentration, NO synthase (NOS) activity, guanylate cyclase activity, NO, and cGMP formation were measured appropriately. The results showed that treatment with SF concentration-dependently inhibited calcium overload induced by anoxia/reoxygenation. We also demonstrated that SF (100-800 microM) concentration dependently enhanced NO and cGMP formation through increasing NOS activity and guanylate cyclase activity in the cardiomyocytes. On the contrary, inhibition of calcium overload by SF was markedly attenuated by addition of an NOS inhibitor, an NO scavenger, an soluble guanylate cyclase inhibitor, and a PKG inhibitor: N(G)-nitro-l-arginine methyl ester (L-NAME, 100 microM), 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazole-1-oxyl-3-oxide (c-PTIO, 1.0 microM), 1H-[1, 2, 4] oxadiazolo [4, 3-alpha] quinoxalin-1-one (ODQ, 20 microM) and KT5823 (0.2 microM), respectively. Our findings indicate that SF significantly attenuates anoxia/reoxygenation-induced Ca(2+) overload and improves cell survival in cultured cardiomyocytes through NO/cGMP/PKG signal pathway.

Antagonistic actions of S(-)-Bay K 8644 on cyclic nucleotide-induced inhibition of voltage-dependent Ba(2+) currents in guinea pig gastric antrum

(+/-)-Bay K 8644, a conventional racemic mixture of Bay K 8644, is widely used as an L-type Ca(2+) channel agonist. Although interactions between Bay K 8644 and cyclic nucleotide have been described, they have not been properly characterized. We have investigated whether two optical isomers of Bay K 8644 (i.e., R(+)- and S(-)-Bay K 8644) modify cyclic nucleotide (cAMP and cGMP)-induced inhibitory effects on nifedipine-sensitive voltage-dependent Ba(2+) currents (I (Ba)) recorded from guinea pig gastric myocytes. Conventional whole-cell recordings were used to compare the effects of R(+)-Bay K 8644 and S(-)-Bay K 8644 on I (Ba). S(-)-Bay K 8644 enhanced the peak amplitude of I (Ba) evoked by depolarizing pulses to +10 mV from a holding potential of -70 mV in a concentration-dependent manner (EC(50) = 32 nM), while R(+)-Bay K 8644 inhibited I (Ba) (IC(50) = 975 nM). When R(+)-Bay K 8644 (0.5 microM) was applied, I (Ba) was suppressed to 71 +/- 10% of control. In the presence of R(+)-Bay K 8644 (0.5 microM), additional application of forskolin and sodium nitroprusside (SNP) further inhibited I (Ba). Conversely, in the presence of S(-)-Bay K 8644 (0.5 microM), subsequent application of forskolin and SNP did not affect I (Ba). Similarly, in the presence of 0.5 microM S(-)-Bay K 8644, db-cAMP and 8-Br-cGMP had no effect on I (Ba). These results indicate that S(-)-Bay K 8644, but not R(+)-Bay K 8644, can prevent the inhibitory actions of two distinct cyclic nucleotide pathways on I (Ba) in gastric myocytes of the guinea pig antrum.

Guanylin peptides and colorectal cancer (CRC)

Agonists of guanylyl-C receptor, such as guanylin/uroguanylin, are correlated not only with the intestinal cell epithelial physiology but also with the colorectal cancer tumorigenesis. Activation of the second intracellular messenger cyclic guanosine monophosphate by guanylyl cyclase-C receptor results in a complex intracellular signalling cascade involving the phosphodiesterase, the ion channels and the protein kinase. After an analytical review of relevant new knowledge, new diagnostic and therapeutic approaches for colorectal cancer are discussed.

Role of L-type calcium channels and PKC in active tone development in rabbit coronary artery

The present study investigated active tone development in isolated ring segments of rabbit epicardial coronary artery. Endothelium-denuded (E-) or endothelium-intact (E+) vessels treated with the NO synthase inhibitor N(omega)-nitro-L-arginine (100 microM) developed active tone, which was enhanced by stretch and reversed by the NO donor sodium nitroprusside (SNP; IC(50)=9 nM). Nifedipine abolished active tone and the contractile response to phorbol dibutyrate (PDBu; 10 nM) with the same potency (IC(50)=8 nM), whereas 300 nM PDBu responses were only partially blocked by nifedipine. The classical and novel PKC inhibitors GF-109203X (IC(50)=1-2 microM) and chelerythrine (IC(50)=4-5 microM) and the classical PKC inhibitor Gö-6976 (IC(50)=0.3-0.4 microM) blocked both active tone and 10 nM PDBu responses with similar potency. Active tone development was associated with depolarization of membrane potential (E(m)) and a shift to the left of the E(m)-vs.-contraction relationship determined by varying extracellular potassium. The depolarization and leftward shift were reversed by either chelerythrine (10 microM) or SNP (30 nM). PDBu (100-300 nM) increased peak L-type calcium channel (Ca(v)) currents in isolated coronary myocytes, and this effect was reversed by chelerythrine (1 microM) or Gö-6976 (200 nM). SNP (500 nM) reduced Ca(v) currents only in the presence of the PKA blocker 8-bromo-2'-O-monobutyryl-cAMPS, Rp isomer (10 microM). In conclusion, active tone development in coronary artery is suppressed by basal NO release and is dependent on both enhanced Ca(v) activity and classical PKC activity. Both E(m)-dependent and -independent processes contribute to contraction. Our results suggest that one E(m)-independent process is direct enhancement of Ca(v) current by PKC.

Study of the mechanisms involved in the vasorelaxation induced by (-)-epigallocatechin-3-gallate in rat aorta

This study investigated several mechanisms involved in the vasorelaxant effects of (-)-epigallocatechin-3-gallate (EGCG). EGCG (1 microM-1 mM) concentration dependently relaxed, after a transient increase in tension, contractions induced by noradrenaline (NA, 1 microM), high extracellular KCl (60 mM), or phorbol 12-myristate 13-acetate (PMA, 1 microM) in intact rat aortic rings. In a Ca2+ -free solution, EGCG (1 microM-1 mM) relaxed 1 microM PMA-induced contractions, without previous transient contraction. However, EGCG (1 microM-1 mM) did not affect the 1 microM okadaic acid-induced contractions. Removal of endothelium and/or pretreatment with glibenclamide (10 microM), tetraethylammonium (2 mM) or charybdotoxin (100 nM) plus apamin (500 nM) did not modify the vasorelaxant effects of EGCG. In addition, EGCG noncompetitively antagonized the contractions induced by NA (in 1.5 mM Ca2+ -containing solution) and Ca2+ (in depolarizing Ca2+ -free high KCl 60 mM solution). In rat aortic smooth muscle cells (RASMC), EGCG (100 microM) reduced increases in cytosolic free Ca2+ concentration ([Ca2+]i) induced by angiotensin II (ANG II, 100 nM) and KCl (60 mM) in 1.5 mM CaCl2 -containing solution and by ANG II (100 nM) in the absence of extracellular Ca2+. In RASMC, EGCG (100 microM) did not modify basal generation of cAMP or cGMP, but significantly reversed the inhibitory effects of NA (1 microM) and high KCl (60 mM) on cAMP and cGMP production. EGCG inhibited the enzymatic activity of all the cyclic nucleotide PDE isoenzymes present in vascular tissue, being more effective on PDE2 (IC50 approximately 17) and on PDE1 (IC50 approximately 25). Our results suggest that the vasorelaxant effects of EGCG in rat aorta are mediated, at least in part, by an inhibition of PDE activity, and the subsequent increase in cyclic nucleotide levels in RASMC, which, in turn, can reduce agonist- or high KCl concentration-induced increases in [Ca2+]i.

Whole-cell recordings of calcium and potassium currents in acutely isolated smooth muscle cells

AIM: To record calcium and potassium currents in acutely isolated smooth muscle cells of mesenteric arterial branches in rats.

METHODS: Smooth muscle cells were freshly isolated by collagenase digest and mechanical trituration with polished pipettes. Patch clamp technique in whole-cell mode was employed to record calcium and potassium currents.

RESULTS: The procedure dissociated smooth muscle cells without impairing the electrophysiological characteristics of the cells. The voltage-gated Ca²⁺ and potassium currents were successfully recorded using whole-cell patch clamp configuration.

CONCLUSION: The method dissociates smooth muscle cells from rat mesenteric arterial branches. Voltage-gated channel currents can be recorded in this preparation.

Nitric oxide synthase inhibition activates L- and T-type Ca2+channels in afferent and efferent arterioles

Previous studies have shown that L-type Ca2+channel (LCC) blockers primarily dilate resting and ANG II-constricted afferent arterioles (AA), but do not influence either resting or ANG II-constricted efferent arterioles (EA). In contrast, blockade of T-type Ca2+channels (TCC) dilate EA and prevent ANG II-mediated efferent constriction. The present study determined the role of LCC and TCC in mediating the AA and EA constriction following inhibition of nitric oxide synthase (NOS) and tested the hypothesis that inhibition of NOS increases the influence of LCC on EA. With the use of an isolated blood-perfused rat juxtamedullary nephron preparation, single AA or EA were visualized and superfused with a NOS inhibitor, N-nitro-l-arginine (l-NNA), with or without concomitant treatment with an LCC blocker, diltiazem, or a TCC blocker, pimozide. In response to l-NNA (1, 10, and 100 μmol/l), AA and EA diameters decreased significantly by 6.0 ± 0.3, 13.7 ± 1.7, and 19.9 ± 1.4%, and by 6.2 ± 0.5, 13.3 ± 1.1, and 19.0 ± 1.9%, respectively. During TCC blockade with pimozide (10 μmol/l), l-NNA did not significantly constrict afferent (0.9 ± 0.6, 1.5 ± 0.5, and 1.7 ± 0.5%) or efferent (0.4 ± 0.1, 2.1 ± 0.7, and 2.5 ± 1.0%) arterioles. In contrast to the responses with other vasoconstictors, the l-NNA-induced constriction of EA, as well as AA, was reversed by diltiazem (10 μmol/l). The effects were overlapping as pimozide superimposed on diltiazem did not elicit further dilation. When the effects of l-NNA were reversed by superfusion with an NO donor, SNAP (10 μmol/l), diltiazem did not cause significant efferent dilation. As a further test of LCC activity, 55 mmol/l KCl, which depolarizes and constricts AA, caused only a modest constriction in resting EA (8.7 ± 1.3%), but a stronger EA constriction during concurrent treatment with l-NNA (23.8 ± 4.8%). In contrast, norepinephrine caused similar constrictions in both l-NNA-treated and nontreated arterioles. These results provide evidence that NO inhibits LCC and TCC activity and that NOS inhibition-mediated arteriolar constriction involves activation of LCC and TCC in both AA and EA. The difference in responses to high KCl between resting and l-NNA-constricted EA and the ability of diltiazem to block EA constriction caused by l-NNA contrasts with the lack of efferent effects in resting and SNAP-treated l-NNA-preconstricted arterioles and during ANG II-mediated vasoconstriction, suggesting a recruitment of LCC in EA when NOS is inhibited. These data help explain how endothelial dysfunction associated with hypertension may lead to enhanced activity of LCC in postglomerular arterioles and increased postglomerular resistance.

Cyclic nucleotide regulation of store-operated Ca2+ influx in airway smooth muscle

Sarcoplasmic reticulum (SR) Ca2+ release and plasma membrane Ca2+ influx are key to intracellular Ca2+ ([Ca2+]i) regulation in airway smooth muscle (ASM). SR Ca2+ depletion triggers influx via store-operated Ca2+ channels (SOCC) for SR replenishment. Several clinically relevant bronchodilators mediate their effect via cyclic nucleotides (cAMP, cGMP). We examined the effect of cyclic nucleotides on SOCC-mediated Ca2+ influx in enzymatically dissociated porcine ASM cells. SR Ca2+ was depleted by 1 microM cyclopiazonic acid in 0 extracellular Ca2+ ([Ca2+]o), nifedipine, and KCl (preventing Ca2+ influx through L-type and SOCC channels). SOCC was then activated by reintroduction of [Ca2+]o and characterized by several techniques. We examined cAMP effects on SOCC by activating SOCC in the presence of 1 microM isoproterenol or 100 microM dibutryl cAMP (cell-permeant cAMP analog), whereas we examined cGMP effects using 1 microM (Z)-1-[N-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (DETA-NO nitric oxide donor) or 100 microM 8-bromoguanosine 3',5'-cyclic monophosphate (cell-permeant cGMP analog). The role of protein kinases A and G was examined by preexposure to 100 nM KT-5720 and 500 nM KT-5823, respectively. SOCC-mediated Ca2+ influx was dependent on the extent of SR Ca2+ depletion, sensitive to Ni2+ and La3+, but not inhibitors of voltage-gated influx channels. cAMP as well as cGMP potently inhibited Ca2+ influx, predominantly via their respective protein kinases. Additionally, cAMP cross-activation of protein kinase G contributed to SOCC inhibition. These data demonstrate that a Ni2+/La3+-sensitive Ca2+ influx in ASM triggered by SR Ca2+ depletion is inhibited by cAMP and cGMP via a protein kinase mechanism. Such inhibition may play a role in the bronchodilatory response of ASM to clinically relevant drugs (e.g., beta-agonists vs. nitric oxide).

Comparative study of the vasorelaxant activity, superoxide-scavenging ability and cyclic nucleotide phosphodiesterase-inhibitory effects of hesperetin and hesperidin

This study investigated the vasorelaxant activity, superoxide radicals (O2(*-))-scavenging capacity and cyclic nucleotide phosphodiesterase (PDE)-inhibitory effects of hesperidin and hesperetin, two flavonoids mainly isolated from citrus fruits. Hesperetin concentration-dependently relaxed the isometric contractions induced by noradrenaline (NA, 1 microM) or by a high extracellular KCl concentration (60 mM) in intact rat isolated thoracic aorta rings. However, hesperetin (10 microM-0.3 mM) did not affect the contractile response induced by okadaic acid (OA, 1 microM). Mechanical removal of endothelium and/or pretreatment of aorta rings with glibenclamide (GB, 10 microM), tetraethylammonium (TEA, 2 mM) or nifedipine (0.1 microM) did not significantly modify the vasorelaxant effects of this flavonoid. Hesperetin (10 microM-0.1 mM) did not affect the basal uptake of (45)Ca(2+) but decreased the influx of (45)Ca(2+) induced by NA and KCl in endothelium-containing and endothelium-denuded rat aorta. Hesperetin (10 microM-0.1 mM) did not scavenge O2(*-) generated by the phenazine methosulfate (PMS)-reduced beta-nicotinamide adenine dinucleotide (NADH) system. Hesperetin (0.1 mM) significantly reversed the inhibitory effects of NA (1 microM) and high KCl (60 mM) on cyclic nucleotide (cAMP and cGMP) production in cultured rat aortic myocytes. Hesperetin preferentially inhibited calmodulin (CaM)-activated PDE1 and PDE4 isolated from bovine aorta with IC(50) values of about 74 microM and 70 microM respectively. In contrast, the 7-rhamnoglucoside of hesperetin, hesperidin (10 microM-0.1 mM), was inactive in practically all experiments, although it inhibited basal and cGMP-activated PDE2 isolated from platelets (IC(50) values of 32+/-4 microM and 137+/-34 microM respectively). These results suggest that the vasorelaxant effects of hesperetin are basically due to the inhibition of PDE1 and PDE4 activities.

Reciprocal regulation and integration of signaling by intracellular calcium and cyclic GMP

Calcium and guanosine-3',5'-cyclic monophosphate (cGMP) are second messenger molecules that regulate opposing physiological functions, reflected in the reciprocal regulation of their intracellular concentrations, in many systems. Indeed, cGMP and Ca2+ constitute discrete points of integration between multiple cell signaling cascades in both convergent and parallel pathways. This chapter describes the molecular mechanisms regulating intracellular Ca2+ and cGMP, and their integration in specific cellular responses.

Adenosine Induces Vasoconstriction through Gi-Dependent Activation of Phospholipase C in Isolated Perfused Afferent Arterioles of Mice

ABSTRACT. Adenosine induces vasoconstriction of renal afferent arterioles through activation of A1 adenosine receptors (A1AR). A1AR are directly coupled to Gi/Go, resulting in inhibition of adenylate cyclase, but the contribution of this signaling pathway to smooth muscle cell activation is unclear. In perfused afferent arterioles from mouse kidney, adenosine and the A1 agonistN6-cyclohexyladenosine, when added to the bath, caused constriction in the concentration range of 10−9to 10−6M (mean diameter: control, 8.8 ± 0.3 μm; adenosine at 10−6M, 2.8 ± 0.5 μm). Adenosine-induced vasoconstriction was stable for up to 30 min and was most pronounced in the most distal part of the afferent arterioles. Adenosine did not cause vasoconstriction in arterioles from A1AR−/− mice. Pretreatment with pertussis toxin (PTX) (400 ng/ml) for 2 h blocked the vasoconstricting action of adenosine orN6-cyclohexyladenosine. PTX pretreatment did not affect the constriction response to KCl, whereas the angiotensin II dose-response relationship was shifted rightward. Reverse transcription-PCR revealed expression of Gi but not Go in kidney cortex and preglomerular vessels. The phospholipase C inhibitor U73122 (4 μM) blocked the constriction responses to both adenosine and angiotensin II. In contrast, the adenylate cyclase inhibitor SQ22536 (10 μM) and the protein kinase A antagonist KT5720 (0.1 and 1 μM) did not induce significant vasoconstriction of afferent arterioles. It is concluded that the constriction response to adenosine in afferent arterioles is mediated by A1AR coupled to a PTX-sensitive Gi protein and subsequent activation of phospholipase C, presumably through βγ subunits released from Gαi. E-mail: jurgens@intra.niddk.nih.gov

Vasodilator actions of abnormal-cannabidiol in rat isolated small mesenteric artery

1. The nonpsychoactive cannabinoid abnormal-cannabidiol (trans-4-[3-methyl-6-(1-methylethenyl)-2-cyclohexen-1-yl]-5-pentyl-1,3-benzenediol) (abn-cbd) produced concentration-dependent relaxation of methoxamine-precontracted rat small mesenteric artery. Endothelial removal reduced abn-cbd potency six-fold without affecting the maximum relaxation. 2. In endothelium-intact vessels, abn-cbd was less potent under 60 mM KCl-induced tone and inhibited by combination of L-N(G)-nitroarginine methyl ester (L-NAME) (nitric oxide synthase inhibitor; 300 micro M), apamin (small conductance Ca(2+)-activated K(+) channels inhibitor; 50 nM) and charybdotoxin (inhibitor of intermediate conductance Ca(2+)-activated K(+) channels and large conductance Ca(2+)-activated K(+) channels BK(Ca); 50 nM). L-NAME alone or in combination with either toxin alone had little effect. 3. In intact vessels, relaxations to abn-cbd were inhibited by SR 141716A (cannabinoid receptor antagonist; 1 or 3 micro M). Concomitant addition of L-NAME, apamin and charybdotoxin had no further effect. Other cannabinoid receptor antagonists either had little (SR 144528; 1 micro M and AM 251; 1 micro M) or no effect (AM 630; 10 micro M and AM 281; 1 micro M). Inhibition of gap junctions, G(i/o) protein coupling and protein kinase A also had no effect. 4. Endothelium-independent relaxation to abn-cbd was unaffected by L-NAME, apamin plus charybdotoxin or capsaicin (10 micro M). Abn-cbd inhibited CaCl(2)-induced contractions in vessels with depleted intracellular Ca(2+) stores and stimulated with methoxamine or KCl. This was insensitive to SR 141716A (3 micro M) but greatly reduced in vessels stimulated with ionomycin (Ca(2+) ionophore; 1 micro M). 5. We conclude that abn-cbd relaxes the rat small mesenteric artery by endothelium-dependent activation of K(+) channels via SR 141716A-sensitive pathways, which do not involve CB(1) and CB(2) receptors. It also causes endothelium-independent, SR 141716A-insensitive, relaxation by inhibiting Ca(2+) entry through voltage-gated Ca(2+) channels.

Enhanced [Ca2+]i in renal arterial smooth muscle cells of pregnant rats with reduced uterine perfusion pressure

Reduction of uterine perfusion pressure (RUPP) during late pregnancy has been suggested to trigger increases in renal vascular resistance and lead to hypertension of pregnancy. We investigated whether the increased renal vascular resistance associated with RUPP in late pregnancy reflects increases in intracellular Ca(2+) concentration ([Ca(2+)](i)) and contraction of renal arterial smooth muscle. Single smooth muscle cells were isolated from renal interlobular arteries of normal pregnant Sprague-Dawley rats and a rat model of RUPP during late pregnancy. The cells were loaded with fura 2 and both cell length and [Ca(2+)](i) were measured. In cells of normal pregnant rats incubated in Hanks' solution (1 mM Ca(2+)), ANG II (10(-7) M) caused an initial increase in [Ca(2+)](i) to 414 +/- 13 nM, a maintained increase to 149 +/- 8 nM, and 21 +/- 1% cell contraction. In RUPP rats, the initial ANG II-induced [Ca(2+)](i) (431 +/- 18 nM) was not different from pregnant rats, but both the maintained [Ca(2+)](i) (225 +/- 9 nM) and cell contraction (48 +/- 2%) were increased. Membrane depolarization by 51 mM KCl and the Ca(2+) channel agonist BAY K 8644 (10(-6) M), which stimulate Ca(2+) entry from the extracellular space, caused maintained increases in [Ca(2+)](i) and cell contraction that were greater in RUPP rats than control pregnant rats. In Ca(2+)-free (2 mM EGTA) Hanks' solution, the ANG II- and caffeine (10 mM)-induced [Ca(2+)](i) transient and cell contraction were not different between normal pregnant and RUPP rats, suggesting no difference in Ca(2+) release from the intracellular stores. The enhanced maintained ANG II-, KCl- and BAY K 8644-induced [Ca(2+)](i) and cell contraction in RUPP rats compared with normal pregnant rats suggest enhanced Ca(2+) entry mechanisms of smooth muscle contraction in resistance renal arteries and may explain the increased renal vascular resistance associated with hypertension of pregnancy.

Functional interplay between angiotensin II and nitric oxide: cyclic GMP as a key mediator

Angiotensin II (Ang II) and nitric oxide (NO) signaling pathways mutually regulate each other by multiple mechanisms. Ang II regulates the expression of NO synthase and NO production, whereas NO downregulates the Ang II type I (AT1) receptor. In addition, downstream effectors of Ang II and NO signaling pathways also interact with each other. A feedback mechanism between Ang II and NO is critical for normal vascular structure and function. Imbalance of Ang II and NO has been implicated in the pathophysiology of many vascular diseases. In this review, we focus on the diverse ways in which Ang II and NO interact and the importance of the balance between the signaling pathways activated by these mediators.

Regulation of cardiac and smooth muscle Ca(2+) channels (Ca(V)1.2a,b) by protein kinases

High voltage-activated Ca(2+) channels of the Ca(V)1.2 class (L-type) are crucial for excitation-contraction coupling in both cardiac and smooth muscle. These channels are regulated by a variety of second messenger pathways that ultimately serve to modulate the level of contractile force in the tissue. The specific focus of this review is on the most recent advances in our understanding of how cardiac Ca(V)1.2a and smooth muscle Ca(V)1.2b channels are regulated by different kinases, including cGMP-dependent protein kinase, cAMP-dependent protein kinase, and protein kinase C. This review also discusses recent evidence regarding the regulation of these channels by protein tyrosine kinase, calmodulin-dependent kinase, purified G protein subunits, and identification of possible amino acid residues of the channel responsible for kinase regulation.

Vascular smooth muscle cell signaling in cirrhosis and portal hypertension

Abnormal vascular responsiveness to ligands has been frequently observed in cirrhosis and portal hypertension, but its existence is not proven. The signaling pathways in vascular smooth muscle cells (VSMCs) have been studied only in animal models of cirrhosis and portal hypertension. Emerging evidence suggests that active relaxation, expressed as augmented content or activity of effectors within the cyclic AMP signaling pathway and suppressed content or activity of effectors in the inositol 1,4,5-trisphosphate/1,2-diacylglycerol signaling pathway, may be occurring in VSMCs of the splanchnic circulation in portal hypertension. The evidence supporting the existence of this phenomenon in the VSMCs of extrasplanchnic circulations in portal hypertension, as well as in the splanchnic circulation when chronic cellular damage is present, is very limited. The status of the other signaling pathways associated with contractile functions of the VSMCs, viz., cyclic GMP and tyrosine kinase-linked pathways, is unknown. The status of all the signaling pathways in non-contractile functions of VSMCs, such as growth and remodeling, has not been studied. As our overall understanding on the signaling pathways in VSMCs is only emerging, it is premature to implicate altered activity of the signaling pathways as the underlying basis of vascular hyporesponsiveness in cirrhosis and portal hypertension, and to extrapolate these limited observations to the human condition.

beta-Adrenergic receptor stimulation of L-type Ca2+ channels in rabbit portal vein myocytes involves both alphas and betagamma G protein subunits

1. Previous studies have shown that purified G protein alphas and betagamma subunits stimulate vascular L-type Ca2+ channels through protein kinase A and C (PKA and PKC), respectively. The present study tested whether activation of endogenous G proteins via beta-adrenergic receptor binding also stimulates vascular Ca2+ channels through both Galphas and Gbetagamma and the subsequent activation of PKA and PKC. 2. Peak Ba2+ current (IBa) in freshly isolated rabbit portal vein smooth muscle cells was significantly increased by bath application of 0.5 microM isoproterenol (isoprenaline; ISO) when measured using the whole-cell patch clamp method (53 +/- 3 % increase, n = 15). Stimulation of IBa by ISO was partially reversed by a PKA inhibitor, KT 5720, or a PKC inhibitor, calphostin C, and completely blocked when cells were pretreated with both KT 5720 and calphostin C. 3. Dialysis of cells with polyclonal antibody to Galphas significantly reduced but did not completely eliminate ISO-induced stimulation of IBa. The remaining stimulation was abolished by calphostin C. Dialysis of cells with a polyclonal antibody to Gbeta also significantly reduced ISO-induced stimulation and the remaining stimulation was abolished by KT 5720. Dialysis of cells with both antibodies completely prevented the stimulation of IBa by ISO. 4. ISO-induced stimulation of IBa was reversed by ICI-118,551, a specific beta2-adrenoceptor antagonist, but not by CGP 20712A, a specific beta1-adrenoceptor antagonist. In addition, the beta2-adrenoceptor agonist zinterol significantly increased peak IBa while the beta1-adrenoceptor agonist dobutamine and beta3-adrenoceptor agonist BRL 37344A had little effect on peak IBa. 5. These data suggest that beta-adrenergic receptor stimulation of vascular L-type Ca2+ channels involves both alphas and betagamma G-protein subunits, which exert their effects through PKA and PKC, respectively.

Iloprost inhibits inositol-1,4,5-trisphosphate-mediated calcium mobilization stimulated by angiotensin II in cultured preglomerular vascular smooth muscle cells

In a previous study of cultured preglomerular vascular smooth muscle cells, it was demonstrated that, although the stable prostacyclin analog iloprost alone had no effect on the intracellular calcium concentration ([Ca2+](i)), it did significantly attenuate the increase in [Ca2+](i) stimulated by angiotensin II (AngII). In this study, the mechanisms by which iloprost interacts with calcium signaling pathways stimulated by AngII were examined. [Ca2+](i) was assessed using the calcium-sensitive fluorescent dye fura-2. Initial studies identified two major components of the [Ca2+](i) response to AngII in this homogeneous preparation of vascular smooth muscle cells from renal resistance vessels. Mobilization of internal stores was evident as an immediate TMB-8-sensitive peak increase in [Ca2+](i) (52 +/- 6 to 297 +/- 26 nM, P: < 0.001) in a calcium-free medium. After [Ca2+](i) had returned to baseline levels during continued AngII stimulation, a nifedipine-sensitive entry pathway was revealed by the sustained stimulatory effect of added external calcium, which increased [Ca2+](i) to 112 +/- 13 nM (P: < 0.001). Coadministration of iloprost with AngII attenuated both the immediate peak (154 +/- 14 nM) and sustained plateau (61 +/- 9 nM) phases. Increases in endogenous levels of cAMP induced by the phosphodiesterase inhibitor milrinone mirrored the actions of iloprost, suggesting that the prostacyclin analog exerted its actions via cAMP activation. Blockade of cAMP-dependent protein kinase with KT 5720 reversed the effects of both iloprost and milrinone. When iloprost or milrinone was introduced after the initial mobilization peak had dissipated, the plateau phase of calcium entry was unchanged (92 +/- 9 nM). The concept that iloprost does not directly modulate calcium entry was further supported by data showing that the activation of L-type calcium channels by BAY-K 8644 was unchanged during iloprost treatment. On the basis of the observation that iloprost did not alter thapsigargin stimulation of Ca(2+)-ATPase activity, it is concluded that the actions of cAMP are distinct from increasing calcium uptake into the sarcoplasmic reticulum. This study provides new information on the ability of iloprost to primarily attenuate inositol-1,4,5-triphosphate-mediated calcium mobilization via cAMP, with secondary inhibition of L-type calcium entry channels. These data clarify the mechanism by which prostaglandins buffer AngII constriction in resistance arterioles.

[Ca(2+)](i) signaling in renal arterial smooth muscle cells of pregnant rat is enhanced during inhibition of NOS

Vascular resistance and arterial pressure are reduced during normal pregnancy, but dangerously elevated during pregnancy-induced hypertension (PIH), and changes in nitric oxide (NO) synthesis have been hypothesized as one potential cause. In support of this hypothesis, chronic inhibition of NO synthesis in pregnant rats has been shown to cause significant increases in renal vascular resistance and hypertension; however, the cellular mechanisms involved are unclear. We tested the hypothesis that the pregnancy-associated changes in renal vascular resistance reflect changes in contractility and intracellular Ca(2+) concentration ([Ca(2+)](i)) of renal arterial smooth muscle. Smooth muscle cells were isolated from renal interlobular arteries of virgin and pregnant Sprague-Dawley rats untreated or treated with the NO synthase inhibitor nitro-L-arginine methyl ester (L-NAME; 4 mg. kg(-1). day(-1) for 5 days), then loaded with fura 2. In cells of virgin rats incubated in Hanks' solution (1 mM Ca(2+)), the basal [Ca(2+)](i) was 86 +/- 6 nM. Phenylephrine (Phe, 10(-5) M) caused a transient increase in [Ca(2+)](i) to 417 +/- 11 nM and maintained an increase to 183 +/- 8 nM and 32 +/- 3% cell contraction. Membrane depolarization by 51 mM KCl, which stimulates Ca(2+) entry from the extracellular space, caused maintained increase in [Ca(2+)](i) to 292 +/- 12 nM and 31 +/- 2% contraction. The maintained Phe- and KCl-induced [Ca(2+)](i) and contractions were reduced in pregnant rats but significantly enhanced in pregnant rats treated with L-NAME. Phe- and KCl-induced contraction and [Ca(2+)](i) were not significantly different between untreated and L-NAME-treated virgin rats or between untreated and L-NAME + L-arginine treated pregnant rats. In Ca(2+)-free Hanks', application of Phe or caffeine (10 mM), to stimulate Ca(2+) release from the intracellular stores, caused a transient increase in [Ca(2+)](i) and a small cell contraction that were not significantly different among the different groups. Thus renal interlobular smooth muscle of normal pregnant rats exhibits reduction in [Ca(2+)](i) signaling that involves Ca(2+) entry from the extracellular space but not Ca(2+) release from the intracellular stores. The reduced renal smooth muscle cell contraction and [Ca(2+)](i) in pregnant rats may explain the decreased renal vascular resistance associated with normal pregnancy, whereas the enhanced cell contraction and [Ca(2+)](i) during inhibition of NO synthesis in pregnant rats may, in part, explain the increased renal vascular resistance associated with PIH.

Effect of nitro derivatives on electromechanical coupling in ureteral smooth muscle cells

Double sucrose gap experiments revealed differences in the effect of nitroglycerin and sodium nitroprusside on action potential and contraction of ureteral smooth muscle cells. Unlike sodium nitroprusside, nitroglycerin inhibited voltage-dependent Ca(2+) membrane permeability. It was concluded that cGMP-independent mechanisms of the effects of nitro derivative reflect the peculiarities of excitation-contraction coupling in smooth muscles.

Mode of nitric oxide action on the renal vasculature

Our study aimed to characterize the essential cellular pathways along which nitric oxide (NO) exerts its well-known vasodilatatory properties in the kidney. Using the isolated perfused rat kidney model we examined the roles of potassium channels, cGMP-protein kinase activity and cAMP-phosphodiesterases (PDE) in the effect of NO on renovascular resistance. We found that neither potassium channel activity nor G-kinase activity was essential for the vasodilatatory effect of NO. The effect of NO, however, was essentially mimicked by pharmacological inhibition of PDE-3, which is a cGMP-inhibitable PDE. As PDE-3 is strongly expressed in renal preglomerular vessels and NO stimulates cGMP formation in renal vessels, it appears likely that inhibition of cAMP degradation and consequently the cAMP pathway are crucially involved in mediating the effects of NO on renal vascular resistance.

Involvement of phosphorylation of beta-subunit in cAMP-dependent activation of L-type Ca2+ channel in aortic smooth muscle-derived A7r5 cells

We investigated the effect of intracellular cAMP on the gating kinetics of L-type Ca2+ channel in an A7r5 smooth muscle-derived cell line using the whole-cell patch-clamp technique. Application of dibutyryl cyclic AMP (db-cAMP) to the cell increased the magnitude of Ca2+ currents through L-type Ca2+ channels (I(Ca)), and shifted the current-voltage relationship (I-V curve) for I(Ca) to the left. The magnitudes of maximum I(Ca) were 14.1 +/- 0.7 before and 16.0 +/- 1.1 pA/pF after application of 1 mM db-cAMP (P < 0.05). The values of the half-activation potential (V(1/2)) of I(Ca), estimated from activation curves, were -7.0 +/- 0.8 mV before and -10.8 +/- 1.0 mV after application of db-cAMP (P < 0.05). In cells pretreated with 10 microM Rp-cAMPS (a specific inhibitor of PKA), db-cAMP affected neither the I-V curve nor the activation curve for I(Ca). In cells pretreated with the antisense oligonucleotide for the beta-subunit of L-type Ca2+ channel, db-cAMP failed to enhance I(Ca) or alter the activation curve. On the other hand, in the cells pretreated with the nonsense oligonucleotide, application of db-cAMP caused an increase in magnitude of I(Ca) and shifted the activation curve to the left. Western blot analysis revealed that the pretreatment of cells with antisense oligonucleotide but nonsense oligonucleotide reduced the expression of the beta-subunit of the L-type Ca2+ channel. We conclude that the cAMP-dependent phosphorylation of the beta-subunit potentiates the voltage dependency of the activation kinetics of the L-type Ca2+ channel in A7r5 cells.

Anchoring protein is required for cAMP-dependent stimulation of L-type Ca(2+) channels in rabbit portal vein

Stimulation of cardiac L-type Ca(2+) channels by cAMP-dependent protein kinase (PKA) requires anchoring of PKA to a specific subcellular environment by A-kinase anchoring proteins (AKAP). This study evaluated the possible requirement of AKAP in PKA-dependent regulation of L-type Ca(2+) channels in vascular smooth muscle cells using the conventional whole cell patch-clamp technique. Peak Ba(2+) current in freshly isolated rabbit portal vein myocytes was significantly increased by superfusion with either 0.5 microM isoproterenol (131 +/- 3% of the control value, n = 11) or 10 microM 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP; 114 +/- 1%, n = 8). The PKA-induced stimulatory effects of both isoproterenol and 8-BrcAMP were completely abolished by a specific PKA inhibitor KT-5720 (0.2 microM) or by dialyzing cells with Ht 31 (100 microM), a peptide that inhibits the binding of PKA to AKAP. In contrast, Ht 31 did not block the excitatory effect of the catalytic subunit of PKA when dialyzed into the cells. These data suggest that stimulation of Ca(2+) channels in vascular myocytes by endogenous PKA requires localization of PKA through binding to AKAP.

Regulation of L-type Ca2+ channels in rabbit portal vein by G protein alphas and betagamma subunits

1. The effect of purified G protein subunits alphas and betagamma on L-type Ca2+ channels in vascular smooth muscle and the possible pathways involved were investigated using freshly isolated smooth muscle cells from rabbit portal vein and the whole-cell patch clamp technique. 2. Cells dialysed with either Galphas or Gbetagamma exhibited significant increases in peak Ba2+ current (IBa) density (148 % and 131 %, respectively) compared with control cells. The combination of Galphas and Gbetagamma further increased peak IBa density (181 %). Inactive Galphas and Gbetagamma did not have any effect on Ca2+ channels. 3. The stimulatory effect of Galphas on peak IBa was entirely abolished by the protein kinase A inhibitor Rp-8-Br-cAMPS, or the adenylyl cyclase inhibitor SQ 22536. On the other hand, the stimulatory response of Ca2+ channels to Gbetagamma was not affected by the protein kinase A inhibitors Rp-8-Br-cAMPS and KT 5720, or by the Ca2+-dependent protein kinase C inhibitor bisindolylmaleimide 1, but was completely blocked by the protein kinase C inhibitor calphostin C. Pretreatment of cells with phorbol 12-myristate 13-acetate for over 18 h prevented the stimulatory effect of Gbetagamma on peak IBa. In addition, acute application of phorbol 12,13-dibutyrate enhanced peak IBa density in control cells, which could be entirely blocked by calphostin C. 4. These data indicate that enhancement of Ba2+ currents by Galphas and Gbetagamma can be attributed to increased activity of protein kinase A and protein kinase C, respectively. No direct membrane-delimited pathway for Ca2+ channel regulation by activated Gs proteins could be detected in vascular smooth muscle cells.

Signaling mechanisms underlying the vascular myogenic response

The vascular myogenic response refers to the acute reaction of a blood vessel to a change in transmural pressure. This response is critically important for the development of resting vascular tone, upon which other control mechanisms exert vasodilator and vasoconstrictor influences. The purpose of this review is to summarize and synthesize information regarding the cellular mechanism(s) underlying the myogenic response in blood vessels, with particular emphasis on arterioles. When necessary, experiments performed on larger blood vessels, visceral smooth muscle, and even striated muscle are cited. Mechanical aspects of myogenic behavior are discussed first, followed by electromechanical coupling mechanisms. Next, mechanotransduction by membrane-bound enzymes and involvement of second messengers, including calcium, are discussed. After this, the roles of the extracellular matrix, integrins, and the smooth muscle cytoskeleton are reviewed, with emphasis on short-term signaling mechanisms. Finally, suggestions are offered for possible future studies.

Modulation of Ca2+ channels by cyclic nucleotide cross activation of opposing protein kinases in rabbit portal vein

Cyclic nucleotides are known to modify voltage-gated (L-type) Ca2+ channel activity in vascular smooth muscle cells, but the exact mechanism(s) underlying these effects is not well defined. The purpose of the present study was to investigate the modulatory role of the cAMP- and cGMP-dependent protein kinase (PKA and PKG, respectively) pathways in Ca2+ channel function by using both conventional and perforated-patch-clamp techniques in rabbit portal vein myocytes. The membrane-permeable cAMP derivative, 8-bromo cAMP (0.1 to 10 micromol/L), significantly increased (14% to 16%) peak Ba2+ currents, whereas higher concentrations (0.05 to 0.1 mmol/L) decreased Ba2+ currents (23% to 31%). In contrast, 8-bromo cGMP inhibited Ba2+ currents at all concentrations tested (0.01 to 1 mmol/L). Basal Ca2+ channel currents were significantly inhibited by the PKA blocker 8-Bromo-2'-O-monobutyryladenosine-3',5'-monophosphorothioate, Rp-isomer (Rp 8-Br-MP cAMPS, 30 micromol/L) and enhanced by the PKG inhibitor beta-Phenyl-1,N2-etheno-8-bromoguanosine-3',5'-monophosphorothioate, Rp-isomer (Rp-8-Br PET cGMPS, 10 nmol/L). In the presence of Rp 8-bromo PET cGMPS (10 to 100 nmol/L), both 8-bromo cAMP (0.1 mmol/L) and 8-bromo cGMP (0.1 mmol/L) enhanced Ba2+ currents (13% to 39%). The excitatory effect of 8-bromo cGMP was blocked by Rp 8-bromo MB-cAMPS. Both 8-bromo cAMP (0.05 mmol/L) and forskolin (10 micromol/L) elicited time-dependent effects, including an initial enhancement followed by suppression of Ba2+ currents. Ba2+ currents were also enhanced when cells were dialyzed with the catalytic subunit of PKA. This effect was reversed by the PKA blocker KT 5720 (200 nmol/L). Our results suggest that cAMP/PKA stimulation enhances and cGMP/PKG stimulation inhibits L-type Ca2+ channel activity in rabbit portal vein myocytes. Our results further suggest that both cAMP and cGMP have a primary action mediated by their own kinase as well as a secondary action mediated by the opposing kinase.