Differential beta-adrenergic regulation and phenotypic modulation of voltage-gated calcium currents in rat aortic myocytes

Dihydropyridine calcium channel blockers and renal disease

Although blood pressure control is considered the main mechanism for preventing the progression of chronic kidney disease (CKD), angiotensin-converting enzyme inhibitors and angiotensin receptors blockers have an additional organ-protective role. The effects of calcium channel blockers (CCBs) in renal disease are not so clearly defined. CCBs have pleiotropic effects that might contribute to protection of the kidney, such as attenuating the mesangial entrapment of macromolecules, countervailing the mitogenic effect of platelet-derived growth factors and platelet-activating factors and suppressing mesangial cell proliferation. Some evidence has accumulated in recent years demonstrating that the new dihydropyridinic CCBs (such as lercanidipine or efonidipine) may affect both postglomerular and preglomerular vessels, resulting in a decreased filtration fraction and nephroprotective effect. Increasing clinical and experimental evidence supports this view and the use of CCBs in CKD hypertensive patients.

L-type Ca(2+) channel current characteristics are preserved in rat tail artery myocytes after one-day storage

AIM

To develop a cheap and simple method of storing for 24-h vascular tissue and single myocytes while preserving therein the biophysical and pharmacological characteristics of L-type Ca(2+) channels and contractile activity.

METHODS

Rings or vascular smooth muscle cells obtained from the rat tail main artery were used either freshly (R0h and VSMC0h) or stored for 24 h (R24h and VSMC24h) at 4 °C, to record whole-cell L-type Ca(2+) currents (IC a(L) ) or measure contractile responses.

RESULTS

R0h/VSMC0h and R24h/VSMC24h comparably contracted when stimulated with phenylephrine, high KCl or ATP. In both VSMC0h and VSMC24h, IC a(L) was identified and characterized as a stable inward current for at least 35 min; IC a(L) was comparably inhibited by the Ca(2+) antagonists nifedipine, verapamil and diltiazem and increased by the Ca(2+) channel agonist (S)-(-)-Bay K 8644; current density and current-voltage relationships were similar; at more hyperpolarized holding potentials, IC a(L) intensity increased comparably; nifedipine shifted the steady-state inactivation curve towards more negative potentials, while verapamil blocked IC a(L) in a frequency-dependent manner and slowed down the rate of recovery from inactivation in a comparable way.

CONCLUSION

Findings show that smooth muscle contractile activity and the biophysical and pharmacological features of L-type Ca(2+) channels are similar in VSMC24h and VSMC0h. The fact that reproducible results were obtained in vascular myocytes up to 24 h after dissociation may facilitate vascular smooth muscle cell investigation by increasing throughput and reducing the number of animals required.

New mechanisms of pulmonary arterial hypertension: role of Ca²⁺ signaling

Pulmonary arterial hypertension (PAH) is a severe and progressive disease that usually culminates in right heart failure and death if left untreated. Although there have been substantial improvements in our understanding and significant advances in the management of this disease, there is a grim prognosis for patients in the advanced stages of PAH. A major cause of PAH is increased pulmonary vascular resistance, which results from sustained vasoconstriction, excessive pulmonary vascular remodeling, in situ thrombosis, and increased pulmonary vascular stiffness. In addition to other signal transduction pathways, Ca(2+) signaling in pulmonary artery smooth muscle cells (PASMCs) plays a central role in the development and progression of PAH because of its involvement in both vasoconstriction, through its pivotal effect of PASMC contraction, and vascular remodeling, through its stimulatory effect on PASMC proliferation. Altered expression, function, and regulation of ion channels and transporters in PASMCs contribute to an increased cytosolic Ca(2+) concentration and enhanced Ca(2+) signaling in patients with PAH. This review will focus on the potential pathogenic role of Ca(2+) mobilization, regulation, and signaling in the development and progression of PAH.

Limits of isolation and culture: intact vascular endothelium and BKCa

The potential physiological role of plasmalemmal large-conductance calcium-activated potassium channels (BKCa) in vascular endothelial cells is controversial. Studies of freshly isolated and cultured vascular endothelial cells provide disparate results, both supporting and refuting a role for BKCa in endothelial function. Most studies using freshly isolated, intact, healthy arteries provide little support for a physiological role for BKCa in the endothelium, although recent work suggests that this may not be the case in diseased vessels. In isolated and cultured vascular endothelial cells, the autocrine action of growth factors, hormones, and vasoactive substances results in phenotypic drift. Such an induced heterogeneity is likely a primary factor accounting for the apparent differences, and often enhanced BKCa expression and function, in isolated and cultured vascular endothelial cells. In a similar manner, heterogeneity in endothelial BKCa expression and function in intact arteries may be representative of normal and disease states, BKCa being absent in normal intact artery endothelium and upregulated in disease where dysfunction induces signals that alter channel expression and function. Indeed, in some intact vessels, there is evidence for the presence of BKCa, such as mRNA and/or specific BK subunits, an observation that is consistent with the potential for rapid upregulation, as may occur in disease. This perspective proposes that the disparity in the results obtained for BKCa expression and function from freshly isolated and cultured vascular endothelial cells is largely due to variability in experimental conditions and, furthermore, that the expression of BKCa in intact artery endothelium is primarily associated with disease. Although answers to physiologically relevant questions may only be available in atypical physiological conditions, such as those of isolation and culture, the limitations of these methods require open and objective recognition.

Vascular effects of calcium channel antagonists: new evidence

Calcium channel antagonists have a well-established role in the management of cardiovascular diseases. L-type calcium channels in vascular cells are a key therapeutic target in hypertension and are the preferred molecular target of the initial calcium channel antagonists. However, third-generation dihydropyridine (DHP) calcium channel antagonists, including manidipine, nilvadipine, benidipine and efonidipine, appear to have effects in addition to blockade of the L-type calcium channel. Voltage-gated calcium channels are widely expressed throughout the cardiovascular system. They constitute the main route for calcium entry, essential for the maintenance of contraction. Cardiac and vascular cells predominantly express L-type calcium channels. More recently, T-type channels have been discovered, and there is emerging evidence of their significance in the regulation of arterial resistance. A lack of functional expression of L-type channels in renal efferent arterioles may be consistent with an important role of T-type channels in the regulation of efferent arteriolar tone. Although the exact role of T-type calcium channels in vascular beds remains to be determined, they could be associated with gene-activated cell replication and growth during pathology. The three major classes of calcium channel antagonists are chemically distinct, and exhibit different functional effects depending on their biophysical, conformation-dependent interactions with the L-type calcium channel. The DHPs are more potent vasodilators, and generally have less cardiodepressant activity than representatives of other classes of calcium channel antagonist such as diltiazem (a phenylalkylamine) and verapamil (a benzothiazepine). In contrast to older calcium channel antagonists, the newer DHPs, manidipine, nilvadipine, benidipine and efonidipine, dilate not only afferent but also efferent renal arterioles, a potentially beneficial effect that may improve glomerular hypertension and provide renoprotection. The underlying mechanisms for the heterogenous effects of calcium channel antagonists in the renal microvasculature are unclear. A credible hypothesis suggests a contribution of T-type calcium channels to efferent arteriolar tone, and that manidipine, nilvadipine and efonidipine inhibit both L and T-type channels. However, other mechanisms, including an effect on neuronal P/Q-type calcium channels (recently detected in arterioles), the microheterogeneity of vascular beds, and other types of calcium influx may also play a role. This article presents recent data about the expression and physiological role of calcium channels in arteries and the molecular targets of the calcium channel antagonists, particularly those exhibiting distinct renovascular effects.

Mechanisms responsible for the in vitro relaxation of ligustrazine on porcine left anterior descending coronary artery

In this study, we have evaluated the underlying mechanisms responsible for the relaxation response of ligustrazine (2,3,5,6-tetra-methyl-pyrazine; 2,3,5,6-MP) and its structural analogues (2-methyl-pyrazine (2-MP); ethyl-pyrazine (EP); 2,3-di-methyl-pyrazine (2,3-MP); 2,5-di-methyl-pyrazine (2,5-MP); 2,6-di-methyl-pyrazine (2,6-MP) and 2,3,5-tri-methyl-pyrazine (2,3,5-MP)) in porcine left anterior descending coronary artery (tertiary branch, O.D. 2,3,5-MP>EP>2,5-MP>/=2,6-MP>/=2,3-MP>2-MP. Besides, salbutamol and forskolin caused an endothelium-independent relaxation. The relaxation response of ligustrazine, salbutamol and forskolin was blunted in the presence of cis-N-(2-phenylcyclopentyl) azacyclotridec-1-en-2-amine (MDL 12330A) (10 microM, an adenylate cyclase inhibitor) and N-[2-((bromocinnamyl)amino)ethyl]-5-isoquinoline-sulphonamide (H-89, a protein kinase A inhibitor, 3 microM). Patch-clamp, whole-cell electrophysiological studies using single smooth muscle cells of the left anterior descending coronary artery revealed that ligustrazine (300 microM), salbutamol (30 microM) and forskolin (1 microM) inhibited the nifedipine-sensitive L-type Ca(2+) channels, and the inhibitory effect was eradicated by MDL 12330A (10 microM) and H-89 (1 microM). However, neither the Ca(2+)-dependent K(+) channel nor the ATP-dependent K(+) channel was modified by ligustrazine (300 microM). In conclusion, our results indicate that ligustrazine-mediated left anterior descending coronary artery relaxation is due to the activation of adenylate cyclase/protein kinase A cascade and the subsequent inhibition of nifedipine-sensitive, voltage-dependent L-type Ca(2+) channels. However, opening of K(+) channels seems to play no role in mediating the relaxation effect of ligustrazine.

Calcium channel subtypes contributing to acetylcholine release from normal, 4-aminopyridine-treated and myasthenic syndrome auto-antibodies-affected neuromuscular junctions

1 Acetylcholine release at the neuromuscular junction relies on rapid, local and transient calcium increase at presynaptic active zones, triggered by the ion influx through voltage-dependent calcium channels (VDCCs) clustered on the presynaptic membrane. Pharmacological investigation of the role of different VDCC subtypes (L-, N-, P/Q- and R-type) in spontaneous and evoked acetylcholine (ACh) release was carried out in adult mouse neuromuscular junctions (NMJs) under normal and pathological conditions. 2 omega-Agatoxin IVA (500 nM), a specific P/Q-type VDCC blocker, abolished end plate potentials (EPPs) in normal NMJs. However, when neurotransmitter release was potentiated by the presence of the K(+) channel blocker 4-aminopyridine (4-AP), an omega-agatoxin IVA- and omega-conotoxin MVIIC-resistant component was detected. This resistant component was only partially sensitive to 1 micro M omega-conotoxin GVIA (N-type VDCC blocker), but insensitive to any other known VDCC blockers. Spontaneous release was dependent only on P/Q-type VDCC in normal NMJs. However, in the presence of 4-AP, it relied on L-type VDCCs too. 3 ACh release from normal NMJs was compared with that of NMJs of mice passively injected with IgGs obtained from patients with Lambert-Eaton myasthenic syndrome (LEMS), a disorder characterized by a compromised neurotransmitter release. Differently from normal NMJs, in LEMS IgGs-treated NMJs an omega-agatoxin IVA-resistant EPP component was detected, which was only partially blocked by calciseptine (1 micro M), a specific L-type VDCC blocker. 4 Altogether, these data demonstrate that multiple VDCC subtypes are present at the mouse NMJ and that a resistant component can be identified under 'pharmacological' and/or 'pathological' conditions.

Comparative study of effects of isoproterenol and vasoactive intestinal polypeptide on voltage-dependent Ca2+ and Ca(2+)-activated K+ currents in porcine tracheal smooth muscle cells

Effects of isoproterenol (Iso) and vasoactive intestinal polypeptide (VIP) on voltage-dependent Ca2+ channel (ICa) and Ca(2+)-activated K+ channel current (IK-Ca) in porcine tracheal smooth muscle cells were examined. When K+ currents were inhibited using a Cs-rich pipette solution, application of 0.1-1 microM Iso or 1-10 nM VIP increased ICa by 20-30%. On the other hand, IK-Ca elicited upon depolarization and spontaneous transient outward K+ currents (STOCs) recorded at a holding potential of -50 mV were enhanced by 80-100% in the presence of 0.1 microM Iso or 1 nM VIP.

Effects of the novel T-type calcium channel antagonist mibefradil on human myocardial contractility in comparison with nifedipine and verapamil

Mibefradil (Ro 40-5967) is a novel nondihydropyridine calcium antagonist. The aim of our study was to compare the negative inotropic effects of the well-known 1,4-dihydropyridine nifedipine and the phenylalkylamine verapamil with those of mibefradil. Isometric force of contraction in response to these substances was determined in isolated, electrically driven left ventricular papillary muscle strips from failing human hearts (1 Hz, 37 degrees C). The hearts were obtained during cardiac transplantation (n = 9) and mitral valve-replacement operations (n = 9). The calcium antagonists studied significantly (p < 0.05) depressed basal force of contraction in a concentration-dependent manner. The effect started at concentrations > 0.001 microM for nifedipine and > 0.01 microM for verapamil, but only at concentrations > 10 microM for mibefradil. Only in the presence of nifedipine and verapamil was a significant rightward shift of the inotropic concentration--response curves to calcium and a depression of the maximal effects of calcium observed. With respect of the relation between the therapeutic active plasma concentration in vivo and the negative intropic potency in vitro, it became evident that the difference between therapeutically beneficial concentrations and potentially hazardous cardiodepressant activity increases from nifedipine to mibefradil. We conclude that this new generation of calcium antagonists, almost lacking cardiodepressant effects, could lead to a greater therapeutic index and greater safety in the treatment of cardiovascular diseases.

Voltage-gated calcium channel currents in human coronary myocytes. Regulation by cyclic GMP and nitric oxide

Voltage-gated Ca2+ channels contribute to the maintenance of contractile tone in vascular myocytes and are potential targets for vasodilating agents. There is no information available about their nature and regulation in human coronary arteries. We used the whole-cell voltage-clamp technique to characterize Ca2+-channel currents immediately after enzymatic dissociation and after primary culture of coronary myocytes taken from heart transplant patients. We recorded a dihydropyridine-sensitive L-type current in both freshly isolated and primary cultured cells. A T-type current was recorded only in culture. The L- (but not the T-) type current was inhibited by permeable analogues of cGMP in a dose-dependent manner. This effect was mimicked by the nitric oxide-generating agents S-nitroso-N-acetylpenicillamine (SNAP) and 3-morpholinosydnonimine which increased intracellular cGMP. Methylene blue, known to inhibit guanylate cyclase, antagonized the effect of SNAP. Inhibitions by SNAP and cGMP were not additive and seemed to occur through a common pathway. We conclude that (a) L-type Ca2+ channels are the major pathway for voltage-gated Ca2+ entry in human coronary myocytes; (b) their inhibition by agents stimulating nitric oxide and/or intracellular cGMP production is expected to contribute to vasorelaxation and may be involved in the therapeutic effect of nitrovasodilators; and (c) the expression of T-type Ca2+ channels in culture may be triggered by cell proliferation.

The alpha 1E calcium channel exhibits permeation properties similar to low-voltage-activated calcium channels

The physiological and pharmacological properties of the alpha 1E calcium (Ca) channel subtype do not exactly match any of the established categories described for native neuronal Ca currents. Many of the key diagnostic features used to assign cloned Ca channels to their native counterparts, however, are dependent on a number of factors, including cellular environment, beta subunit coexpression, and modulation by second messengers and G-proteins. Here, by examining the intrinsic pore characteristics of a family of transiently expressed neuronal Ca channels, we demonstrate that the permeation properties of alpha 1E closely resemble those described for a subset of low-threshold Ca channels. The alpha 1A (P-/Q-type), alpha 1B (N-type), and alpha 1C (L-type) high-threshold Ca channels all exhibit larger whole-cell currents with barium (Ba) as the charge carrier as compared with Ca or strontium (Sr). In contrast, macroscopic alpha 1E currents are largest in Sr, followed by Ca and then Ba. The unique permeation properties of alpha 1E are maintained at the single-channel level, are independent of the nature of the expression system, and are not affected by coexpression of alpha 2 and beta subunits. Overall, the permeation characteristics of alpha 1E are distinct from those described for R-type currents and share some similarities with native low-threshold Ca channels.

The alpha 1E calcium channel exhibits permeation properties similar to low-voltage-activated calcium channels

The physiological and pharmacological properties of the alpha 1E calcium (Ca) channel subtype do not exactly match any of the established categories described for native neuronal Ca currents. Many of the key diagnostic features used to assign cloned Ca channels to their native counterparts, however, are dependent on a number of factors, including cellular environment, beta subunit coexpression, and modulation by second messengers and G-proteins. Here, by examining the intrinsic pore characteristics of a family of transiently expressed neuronal Ca channels, we demonstrate that the permeation properties of alpha 1E closely resemble those described for a subset of low-threshold Ca channels. The alpha 1A (P-/Q-type), alpha 1B (N-type), and alpha 1C (L-type) high-threshold Ca channels all exhibit larger whole-cell currents with barium (Ba) as the charge carrier as compared with Ca or strontium (Sr). In contrast, macroscopic alpha 1E currents are largest in Sr, followed by Ca and then Ba. The unique permeation properties of alpha 1E are maintained at the single-channel level, are independent of the nature of the expression system, and are not affected by coexpression of alpha 2 and beta subunits. Overall, the permeation characteristics of alpha 1E are distinct from those described for R-type currents and share some similarities with native low-threshold Ca channels.

Bay K 8644 reveals two components of L-type Ca2+ channel current in clonal rat pituitary cells

Whole-cell L-type Ca2+ channel current was recorded in GH3 clonal rat pituitary cells using Ba2+ as a charge carrier. In the presence of the dihydropyridine agonist Bay K 8644, deactivation was best described by two exponential components with time constants of approximately 2 and approximately 8 ms when recorded at -40 mV. The slow component activated at more negative potentials than the fast component: Half-maximal activation for the slow and fast components occurred at approximately -15 and approximately 1 mV, respectively. The fast component was more sensitive to enhancement by racemic Bay K 8644 than the slow component: ED50fast = approximately 21 nM, ED50slow = approximately 74 nM. Thyrotropin-releasing hormone (TRH; 1 microM) inhibited the slow component by approximately 46%, whereas the fast component was inhibited by approximately 22%. TRH inhibition of total L-current showed some voltage dependence, but each Bay K 8644-revealed component of L-current was inhibited in a voltage-independent manner. Therefore, the apparent voltage dependence of TRH action is derived from complexities in channel gating rather than from relief of inhibition at high voltages. In summary, Bay K 8644-enhanced L-currents in GH3 cells consist of two components with different sensitivities to voltage, racemic Bay K 8644, and the neuropeptide TRH.

Absence of calcium channels in neonatal rat aortic myocytes

We have investigated whole-cell Ba2+ currents through Ca2+ channels (IBa) in single myocytes freshly isolated from the aortic media of neonatal (1-day-old) and adult (12-week-old) rats. In neonatal myocytes, (IBa) was undetectable even in presence of the dihydropyridine (DHP) agonist Bay K 8644. Binding of [3H]Nitrendipine on crude plasma membrane preparation of media confirmed the absence of DHP-receptors. By contrast, a robust DHP-sensitive 'L-type' IBa was recorded in adults which was consistent with the presence of specific [3H]Nitrendipine binding sites. In conclusion, neonatal aortic myocytes do not express any Ca2+ channels. The acquisition of L-type Ca2+ channels may be related to cell differentiation and acquisition of contractility during postnatal development.

Effects of 8-bromo cyclic GMP and verapamil on depolarization-evoked Ca2+ signal and contraction in rat aorta

1. The pharmacological action of NO donors is usually attributed to a cellular rise in guanosine 3':5'-cyclic monophosphate (cyclic GMP), but this hypothesis is based only on indirect evidence. Therefore, we have studied the effects of cyclic GMP on Ca2+ movements and contraction in rat isolated endothelium-denuded aorta stimulated by KCl depolarizing solution using the permeant analogue 8-bromo cyclic GMP (BrcGMP). Isometric contraction and fura-2 Ca2+ signals were measured simultaneously in preparations treated with BrcGMP and with verapamil. The activation of calcium channels was estimated by measuring the quenching rate of the intracellular fura-2 signal by Mn2+ and by the depolarization-dependent influx of 45Ca2+. 2. Stimulation with 67 mM KCl-solution evoked an increase in cytosolic Ca2+ concentration ([Ca2+]cyt) and a contractile response which were inhibited by pretreatment with verapamil (0.1 microM) or BrcGMP (0.1-1 mM). However, the inhibition of the fura-2 Ca2+ signal was significantly higher with verapamil than with BrcGMP, whereas the contraction was inhibited to a similar extent. 3. When preparations were exposed to K(+)-depolarizing solution in which the calcium concentration was cumulatively increased, the related increase in fura-2 Ca2+ signal was barely affected by BrcGMP, whereas the contractile tension was strongly and significantly inhibited. 4. Cellular Ca2+ changes were also estimated with 45Ca2+. 45Ca2+ influx in resting preparations was significantly reduced by BrcGMP (0.1 mM) but not by verapamil (0.1 microM); 45Ca2+ influx in KCl-depolarized preparations was reduced by verapamil but was unaffected by BrcGMP. 5. Measurements of Mn2+-induced quenching of the intracellular fura-2 signal showed that BrcGMPdid not affect divalent cation entry in K+-stimulated preparations, whereas verapamil concentration dependently inhibited Mn2+ entry stimulated by K+-depolarization.6. The present results indicate that BrcGMP did not affect voltage-dependent Ca2+ channel gating in the rat aorta. For a given fura-2 Ca2+ signal, the contraction was lower in preparations exposed toBrcGMP than in the untreated ones, suggesting that the activation of cyclic GMP-dependent kinases reduced the contractile efficacy of calcium. Furthermore, the reduction of depolarization-dependent 45Ca2+ uptake reported with sodium nitroprusside, a NO donor, was not observed with biologically active concentrations of BrcGMP, suggesting that this drug could have additional mechanisms of action,unrelated to activation of protein G-kinase.