Protein kinase A increases availability of calcium channels in smooth muscle cells from guinea pig basilar artery

Vascular CaV1.2 channels in diabetes

Diabetic vasculopathy is a significant cause of morbidity and mortality in the diabetic population. Hyperglycemia, one of the central metabolic abnormalities in diabetes, has been associated with vascular dysfunction due to endothelial cell damage. However, studies also point toward vascular smooth muscle as a locus for hyperglycemia-induced vascular dysfunction. Emerging evidence implicates hyperglycemia-induced regulation of vascular L-type Ca²⁺ channels Ca_V1.2 as a potential mechanism for vascular dysfunction during diabetes. This chapter summarizes our current understanding of vascular Ca_V1.2 channels and their regulation during physiological and hyperglycemia/diabetes conditions. We will emphasize the role of Ca_V1.2 in vascular smooth muscle, the effects of elevated glucose on Ca_V1.2 function, and the mechanisms underlying its dysregulation in hyperglycemia and diabetes. We conclude by examining future directions and gaps in knowledge regarding Ca_V1.2 regulation in health and during diabetes.

Ion channels as effectors of cyclic nucleotide pathways: Functional relevance for arterial tone regulation

Numerous mediators and drugs regulate blood flow or arterial pressure by acting on vascular tone, involving cyclic nucleotide intracellular pathways. These signals lead to regulation of several cellular effectors, including ion channels that tune cell membrane potential, Ca²⁺ influx and vascular tone. The characterization of these vasocontrictive or vasodilating mechanisms has grown in complexity due to i) the variety of ion channels that are expressed in both vascular endothelial and smooth muscle cells, ii) the heterogeneity of responses among the various vascular beds, and iii) the number of molecular mechanisms involved in cyclic nucleotide signalling in health and disease. This review synthesizes key data from literature that highlight ion channels as physiologically relevant effectors of cyclic nucleotide pathways in the vasculature, including the characterization of the molecular mechanisms involved. In smooth muscle cells, cation influx or chloride efflux through ion channels are associated with vasoconstriction, whereas K⁺ efflux repolarizes the cell membrane potential and mediates vasodilatation. Both categories of ion currents are under the influence of cAMP and cGMP pathways. Evidence that some ion channels are influenced by CN signalling in endothelial cells will also be presented. Emphasis will also be put on recent data touching a variety of determinants such as phosphodiesterases, EPAC and kinase anchoring, that complicate or even challenge former paradigms.

Adenosine-A2a receptor down-regulates cerebral smooth muscle L-type Ca2+ channel activity via protein tyrosine phosphatase, not cAMP-dependent protein kinase

Adenosine acting via A2a receptors (A2aR) is a potent cerebral vasodilator that relaxes vascular smooth muscle cells (VSMCs) by a mechanism attributed to activation of cAMP-dependent protein kinase (cAK). We examined effects of adenosine and its mechanism of action on L-type Ca2+ channels in native VSMCs from rat basilar artery. Reverse transcription-polymerase chain reaction and immunofluorescence imaging confirmed transcription and expression of A2aR, and in situ hybridization confirmed presence of mRNA for L-type Cav1.2b channels. In patch-clamp experiments, adenosine down-regulated Ca2+ channel currents in a concentration-dependent manner, with receptor-subtype-specific antagonists [4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo-[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM-241385) versus 1,3-dipropyl-8-cyclopentyl-1,3-dipropylxanthine] showing that this was caused by action of A2aR. Down-regulation of channel currents was mimicked by stimulation of cGMP-dependent protein kinase (cGK; 8-Br-cGMP) and by inhibition of tyrosine kinase (AG-18) but not by stimulation of cAK [forskolin and 8-bromo-cAMP (8-Br-AMP)]. Down-regulation of currents by the A2aR agonist 2-[p-(2-carboxyeth yl)phenylethylamino]-5'-N-ethyolcarboxamidoadenosine (CGS-21680) was blocked by inhibiting protein tyrosine phosphatase (PTP; orthovanodate and dephostatin), but not by inhibiting cGK (KT-5823 and H-7). Western blots of lysate or of immunoisolated Ca2+ channels from arterial segments incubated with CGS-21680 showed 1) increased phosphorylation of vasodilator-stimulated phosphoprotein that was blocked by inhibiting cAK (KT-5720), consistent with activation of cAK by A2aR; and 2) decreased tyrosine phosphorylation of immunoisolated alpha1c subunit of the Ca2+ channel. Our data show that cAK, although activated, was not germane to down-regulation of Ca2+ channel activity by A2aR, and they delineate a novel signaling mechanism involving reduced tyrosine phosphorylation of Ca2+ channels by A2aR probably caused by PTP activation.

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. </=1 mm). In 5-hydroxytryptamine (3 microM) precontracted preparations, cumulative administration (0.1-300 microM) of all pyrazine analogues caused an endothelium-independent, concentration-dependent relaxation. The relative inhibitory potency, as compared at concentration with which 50% relaxation occurred, was 2,3,5,6-MP>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.

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.

Chronic nicotine alters NO signaling of Ca(2+) channels in cerebral arterioles

Smoking is a major health hazard with proven deleterious effects on the cerebral circulation, including a decrease in cerebral blood flow and a high risk for stroke. To elucidate cellular mechanisms for the vasoconstrictive and pathological effects of nicotine, we used a nystatin-perforated patch-clamp technique to study Ca(2+) channels and Ca(2+)-activated K(+) (BK) channels in smooth muscle cells isolated from cerebral lenticulostriate arterioles of rats chronically exposed to nicotine (4.5 mg/kg per day of nicotine free base, 15 to 22 days via osmotic minipump). Two major effects were observed in cells from nicotine-treated animals compared with controls. First, Ca(2+) channels were upregulated (0.48+/-0.03 pS/pF [20 cells] versus 0.35+/-0.01 pS/pF [31 cells], P:<0.005) and BK channels were downregulated (12+/-3 pA/pF [14 cells] versus 34+/-7 pA/pF [14 cells], P:<0.05), mimicking the effect of an apparent decrease in bioavailability of endogenous NO. Second, normal downregulation of Ca(2+) channels by exogenous NO (sodium nitroprusside [SNP], 100 nmol/L) and cGMP (8-bromo-cGMP, 0.1 mmol/L) was absent, whereas normal upregulation of BK channels by these agents was preserved, suggesting block of NO signaling downstream of cGMP-dependent protein kinase. In pial window preparations, chronic nicotine blunted NO-induced vasodilation of pial vessels and the increase in cortical blood flow measured by laser-Doppler flowmetry, demonstrating the importance of Ca(2+) channel downregulation in NO-induced vasorelaxation. These findings elucidate a new pathophysiological mechanism involving altered Ca(2+) homeostasis in cerebral arterioles that may predispose to stroke.

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.

Involvement of both G protein alphas and beta gamma subunits in beta-adrenergic stimulation of vascular L-type Ca(2+) channels

1. Previous data have shown that activation of beta(3)-adrenoceptors stimulates vascular L-type Ca(2+) channels through a G alphas-induced stimulation of the cyclic AMP/PKA pathway. The present study investigated whether beta-adrenergic stimulation also uses the G beta gamma/PI3K/PKC pathway to modulate L-type Ca(2+) channels in rat portal vein myocytes. 2. Peak Ba(2+) current (I(Ba)) measured using the whole-cell patch clamp method was maximally increased by application of 10 microm isoprenaline after blockade of beta(3)-adrenoceptors by 1 microM SR59230A. Under these conditions, the isoprenaline-induced stimulation of I(Ba) was reversed by ICI-118551 (a specific beta(2)-adrenoceptor antagonist) but not by atenolol (a specific beta(1)-adrenoceptor antagonist). The beta(2)-adrenoceptor agonist salbutamol increased I(Ba), an effect which was reversed by ICI-118551 whereas the beta(1)-adrenoceptor agonist dobutamine had no effect on I(Ba). 3. Application of PKA inhibitors (H-89 and Rp 8-Br-cyclic AMPs) or a PKC inhibitor (calphostin C) alone did not affect the beta(2)-adrenergic stimulation of I(Ba) whereas simultaneous application of both PKA and PKC inhibitors completely blocked this stimulation. 4. The beta(2)-adrenergic stimulation of L-type Ca(2+) channels was blocked by a pre-treatment with cholera toxin and by intracellular application of an anti-G alphas antibody (directed against the carboxyl terminus of G alphas). In the presence of H-89, intracellular infusion of an anti-Gss(com) antibody or a beta ARK(1) peptide as well as a pre-treatment with wortmannin (a PI3K inhibitor) blocked the beta(2)-adrenergic stimulation of I(Ba). 5. These results suggest that the beta(2)-adrenergic stimulation of vascular L-type Ca(2+) channels involves both G alphas and G beta gamma subunits which exert their stimulatory effects through PKA and PI3K/PKC pathways, respectively.

Beta-3 adrenergic stimulation of L-type Ca(2+) channels in rat portal vein myocytes

1. The effects of beta(3)-adrenergic stimulation were studied on the L-type Ca(2+) channel in single myocytes from rat portal vein using the whole-cell mode of the patch-clamp technique. 2. Reverse transcription-polymerase chain reaction showed that beta(1)-, beta(2)- and beta(3)-adrenoceptor subtypes were expressed in rat portal vein myocytes. Application of both propranolol (a non-selective beta(1)- and beta(2)-adrenoceptor antagonist) and SR59230A (a beta(3)-adrenoceptor antagonist) were needed to inhibit the isoprenaline-induced increase in L-type Ca(2+) channel current. 3. L-type Ca(2+) channels were stimulated by CGP12177A (a beta(3)-adrenoceptor agonist with potent beta(1)- and beta(2)-adrenoceptor antagonist property) in a manner similar to that of isoprenaline. The CGP12177A-induced stimulation of Ca(2+) channel current was blocked by SR59230A, cyclic AMP-dependent protein kinase inhibitors, H-89 and Rp 8-Br-cyclic AMPs, but was unaffected by protein kinase C inhibitors, GF109203X and 19-31 peptide. This stimulation was mimicked by forskolin and 8-Br-cyclic AMP. In the presence of okadaic acid (a phosphatase inhibitor), the beta(3)-adrenoceptor-induced stimulation was maintained after withdrawal of the agonist. 4. The beta(3)-adrenoceptor stimulation of L-type Ca(2+) channels was blocked by a pretreatment with cholera toxin and by the intracellular application of an anti-Galpha(s) antibody. This stimulation was unaffected by intracellular infusion of an anti-Gbeta(com) antibody and a betaARK(1) peptide. 5. These results show that activation of beta(3)-adrenoceptors stimulates L-type Ca(2+) channels in vascular myocytes through a Galpha(s)-induced stimulation of the cyclic AMP/protein kinase A pathway and the subsequent phosphorylation of the channels.

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.

Increase in functional Ca2+ channels in cerebral smooth muscle with renal hypertension

The hypothesis that availability of functional Ca2+ channels in vascular smooth muscle is augmented in hypertension was tested in basilar artery cells from Wistar rats exhibiting stable systolic blood pressure (BPsys) for 2 to 11 weeks after partial renal artery ligation (Goldblatt 2-kidney 1-clip [2K1C] model). Cells were freshly isolated and patch-clamped using a nystatin-perforated patch method. BPsys ranged from 110 to 280 mm Hg and correlated with normalized kidney mass. Macroscopic current-voltage curves were fit to a Boltzmann function to obtain maximum conductance (gmax), steepness and midpoint potential for the voltage dependence of activation (k and E1/2, respectively), and extrapolated reversal potential for the chord conductance (Erev). Linear regression of normalized conductance (ng(max)=g(max)/cell capacitance) versus BPsys for 103 cells indicated a strong relationship, with a slope of 0.0019 nS x pF(-1) x mm Hg(-1) (P<0.0001). Similar analysis of data from 35 other cells exposed to 500 nmol/L Bay K 8644 gave a slope of 0.0041 nS x pF(-1) x mm Hg(-1) (P=0.001). Voltage-dependent parameters, k, E1/2, and Erev, were not significantly related to BPsys. Single-channel measurements in cell-attached patches revealed that the number of channels in 32 patches was significantly related to BPsys (P=0.0024) but that slope conductance, open dwell times at 0 mV, and distribution between 2 open states were not. Finally, in a subgroup of 61 cells from animals made hypertensive (180 mm Hg<BPsys<200 mm Hg) for approximately 1/2 to 6 weeks, we found that elevation of ngmax depended on duration of hypertension (P=0.003), with no elevation at approximately 1/2 week. We conclude that in the 2K1C model, availability of functional Ca2+ channels increases with BPsys with no change in channel properties and that measurable BPsys elevation occurs before the increase in functional channels.

Hypoxia and smooth muscle function: key regulatory events during metabolic stress

Hypoxia rapidly reduces force in many smooth muscles and we review recent data that shed light on the mechanisms involved. As many regulated cellular processes are integrated to co-ordinate smooth muscle contractility, the processes responsible for decreased force output with altered metabolism are also likely to be many, acting in concert, rather than the actions of one altered parameter. Nevertheless the aim of this study is to elucidate the hierarchical series of events that contribute to reduced smooth muscle force production during altered metabolism. We conclude that in many phasic smooth muscles the decrease in force can be attributed to impaired electro-mechanical coupling whereby the Ca2+ transient is reduced. A direct effect of hypoxia on the Ca2+ channel may be of key importance. In tonic vascular smooth muscles KATP channels may also play a role in the integrated functional responses to hypoxia. There are also many examples of force being reduced, in tonically activated preparations, without a fall in steady-state Ca2+; indeed it usually increases. We examine the roles of altered [ATP], pH, myosin phosphorylation, inorganic phosphate and proteolytic activity on the [Ca2+]-force relationship during hypoxia. We find no defining force-inhibitory role for any one factor acting alone, and suggest that force most probably falls as a result of the combination of myriad factors.

Single L-type calcium channels in smooth muscle cells from resistance arteries of spontaneously hypertensive rats

The amplitude of the whole-cell L-type Ca2+ channel current recorded from vascular smooth muscle cells is reportedly greater in spontaneously hypertensive rats (SHR) than in Wistar-Kyoto rats (WKY). However, no study has examined properties of single Ca2+ channels in arterial cells from these strains. To further test the hypothesis that activation of L-type Ca2+ channels in arterial smooth muscle cells would be enhanced in SHR, we recorded single Ca2+ channel currents in resistance mesenteric artery cells from SHR and WKY (8 to 9 weeks of age) using a cell-attached patch clamp technique. With 50 mmol/L Ba2+ in the recording pipette, the depolarizing pulse from a holding potential of -40 mV evoked the single L-type Ca2+ channel current. Opening of the single channels was more frequent in cells from SHR than from WKY. Single-channel conductance (20 pS) and open time (1 ms at 0 mV) did not differ in the two strains. The results suggest that an increased amplitude of the whole-cell current can be attributed to the enhanced opening of single Ca2+ channels in the arterial smooth muscle cells from SHR compared with WKY.

Effect of inhibition of tyrosine phosphatases on voltage-operated calcium channel currents in rabbit isolated ear artery cells

1. The effect of increasing cellular tyrosine phosphorylation by inhibiting endogenous tyrosine phosphatases was examined on voltage-operated calcium channel currents in vascular smooth muscle cells. 2. In single ear artery smooth muscle cells of the rabbit, studied by the whole cell voltage clamp technique, intracellular application of the tyrosine phosphatase inhibitors, sodium orthovanadate (100 microM) and peroxyvanadate (100 microM orthovanadate + 1 mM H2O2) increased voltage-operated calcium channel currents by 56% and 83%, respectively. 3. Bath application of two other membrane permeant tyrosine phosphatase inhibitors, phenylarsine oxide (100 microM) and dephostatin (50 microM) also increased voltage-operated calcium channel currents by 48% and 52%, respectively. 4. The selective tyrosine kinase inhibitor, tyrphostin-23 (100 microM) reduced calcium channel currents by 41%. Pre-incubation with tyrphostin-23 abolished the effects of peroxyvanadate, phenylarsine oxide and dephostatin on calcium channels. 5. Western blot analysis of rabbit ear artery cell lysates showed increased tyrosine phosphorylation of several endogenous proteins following treatment with peroxyvanadate. 6. These results indicate that a number of structurally dissimilar inhibitors of tyrosine phosphatases increase voltage-operated calcium channel currents in arterial smooth muscle cells presumably due to increased tyrosine phosphorylation.

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.

The complement membrane attack complex and the bystander effect in cerebral vasospasm

Activation of complement results in formation of membrane attack complexes (MACs) that can insert themselves either into cells that initiate complement activation or into nearby (“innocent bystander”) cells. The MACs form large-conductance, nonspecific ion channels that can cause lytic or sublytic cell damage. The authors used a highly sensitive patch clamp technique to assess the contribution of the bystander effect to the pathophysiology of cerebral vasospasm. They compared the effect of complement activation by autologous aged versus fresh erythrocytes on the membrane conductance of freshly isolated rat cerebral artery smooth-muscle cells. In the presence of autologous serum, aged, but not fresh, erythrocytes caused a large increase in membrane conductance, an effect that was prevented by heat-inactivating the serum. Ethyleneglycol tetraacetic acid in the presence of Mg++ attenuated the effect, indicating that complement activation was taking place via the classic pathway. The effect was reproduced by zymosan-activated autologous serum, suggesting that such changes in conductance could result from insertion of MACs secondary to a bystander effect. Both C8- and C9-depleted heterologous sera produced minimal effects that were converted to full effect by addition of the missing complement component. Superoxide dismutase plus catalase did not attenuate the conductance changes produced by autologous serum plus aged erythrocytes. Autologous serum plus aged erythrocyte membrane ghosts that were free of lysate caused a typical increase in conductance. This study demonstrates that complement activation by aged erythrocytes can result in MAC insertion into innocent bystander smooth-muscle cell membranes and that this mechanism, heretofore undescribed, may contribute to development of vasospasm after subarachnoid hemorrhage.

Oxidized LDLs but not native LDLs augment Ba2+ currents through L-type Ca2+ channels of the A7r5 smooth muscle-derived cell line

The whole-cell patch-clamp method was used on A7r5 smooth muscle-derived cell line, and Ba2+ currents through Ca2+ channels were recorded. The A7r5 cells showed voltage-dependent, long-lasting Ba2+ currents which were markedly inhibited by nifedipine (10 microM). The magnitude of the maximum Ba2+ current (IBa(max)) was augmented by an application of dbcAMP (1 mM), but not affected by TPA (80 nM). Noradrenaline (NA) at 100 microM caused an increase in the IBa(max) by 19.7% in the presence of phentolamine (10 microM). This effect was cancelled by Rp-cAMPs (10 microM). In the presence of propranolol (10 microM), NA tended to reduce the IBa(max). Application of Ox-LDLs at 100 microg protein/ml caused an increase in the IBa(max) by 15.7%, whereas native LDLs did not change the IBa(max). Rp-cAMPs was ineffective to the Ox-LDL action on the IBa(max). In the presence of Ox-LDLs, NA augmented the IBa(max) by 21.4% in the presence of phentolamine. These results suggest that Ox-LDLs activate L-type Ca2+ channels of A7r5 cells by a mechanism independent of cAMP/PKA signalling.

The complement membrane attack complex and the bystander effect in cerebral vasospasm

Activation of complement results in formation of membrane attack complexes (MACs) that can insert themselves either into cells that initiate complement activation or into nearby ("innocent bystander") cells. The MACs form large-conductance, nonspecific ion channels that can cause lytic or sublytic cell damage. The authors used a highly sensitive patch clamp technique to assess the contribution of the bystander effect to the pathophysiology of cerebral vasospasm. They compared the effect of complement activation by autologous aged versus fresh erythrocytes on the membrane conductance of freshly isolated rat cerebral artery smooth-muscle cells. In the presence of autologous serum aged, but not fresh, erythrocytes caused a large increase in membrane conductance, an effect that was prevented by heat-inactivating the serum. Ethyleneglycol tetraacetic acid in the presence of Mg++ attenuated the effect, indicating that complement activation was taking place via the classic pathway. The effect was reproduced by zymosan-activated autologous serum, suggesting that such changes in conductance could result from insertion of MACs secondary to a bystander effect. Both C8- and C9-depleted heterologous sera produced minimal effects that were converted to full effect by addition of the missing complement component. Superoxide dismutase plus catalase did not attenuate the conductance changes produced by autologous serum plus aged erythrocytes. Autologous serum plus aged erythrocyte membrane ghosts that were free of lysate caused a typical increase in conductance. This study demonstrates that complement activation by aged erythrocytes can result in MAC insertion into innocent bystander smooth-muscle cell membranes and that this mechanism, heretofore undescribed, may contribute to development of vasospasm after subarachnoid hemorrhage.

Actions of neurotransmitters and other messengers on Ca2+ channels and K+ channels in smooth muscle cells

Ion channels play key roles in determining smooth muscle tone by setting the membrane potential and allowing Ca2+ influx. Perhaps not surprisingly, therefore, they also provide targets for neurotransmitters and other messengers that act on smooth muscle. Application of patch-clamp and molecular biology techniques and the use of selective pharmacology has started to provide a wealth of information on the ion channel systems of smooth muscle cells, revealing complexity and functional significance. Reviewed are the actions of messengers (e.g., noradrenaline, acetylcholine, endothelin, angiotensin II, neuropeptide Y, 5-hydroxytryptamine, histamine, adenosine, calcitonin gene-related peptide, substance P, prostacyclin, nitric oxide and oxygen) on specific types of ion channel in smooth muscle, the L-type calcium channel, and the large conductance Ca(2+)-activated, ATP-sensitive, delayed rectifier and apamin-sensitive K+ channels.

Modulation of Ca2+ channel activity by ATP metabolism and internal Mg2+ in guinea-pig basilar artery smooth muscle cells

1. Single smooth muscle cells were isolated from the basilar artery of the guinea-pig and, within 10 h, inward currents through voltage-gated Ca2+ channels were recorded using the amphotericin or conventional whole-cell voltage-clamp techniques. 2. In amphotericin whole-cell recordings, bath application of 2,4-dinitrophenol (DNP, an uncoupler of mitochondrial ATP production) induced an initial stimulation (14% increase in 5 of 11 cells) and then pronounced inhibition (50% decrease in 9 of 11 cells within 9.5 min) of voltage-dependent Ca2+ current (I(Ca)) elicited by depolarizing to +10 mV in 1.5 mM extracellular Ca2+ solution. By contrast, inhibition of glycolysis by replacing glucose in the bath with 2-deoxy-D-glucose had no effect. 3. Na+ current through Ca2+ channels (I[(Ca)(Na)]) recorded in the absence of extracellular divalent cations also responded to DNP, again with stimulation followed by inhibition of current. The stimulation of I[(Ca)(Na)] was associated with a leftward shift of the Ca2+ channel activation curve which averaged -9 mV. A combination of 2-deoxy-D-glucose, mannoheptulose and 3-0-methyl-glucose had only minor effects on I[(Ca)(Na)], whereas rotenone had an effect similar to that of DNP in six of eight cells. 4. The amplitude of I[(Ca)(Na)] in conventional whole-cell recordings was not different from that in amphotericin whole-cell recordings, even without ATP in the recording pipette and with metabolic poisons in the bath solution. Furthermore, attempts to dephosphorylate the Ca2+ channels in ATP-free conditions did not prevent I[(Ca)(Na)], and a high concentration of Mg-ATP with or without a phosphorylation-supporting medium in the recording pipette did not increase its amplitude. 5. In the absence of ATP, Mg2+ inhibited whole-cell I[Ca)(Na)] with a K(d) of about 100 mu M at -10 mV and induced a leftward shift of the Ca2+ channel activation curve. When ATP and a phosphorylation-supporting medium were in the recording pipette the blocking effect of free Mg2+ was reduced but the shift in the Ca2+ channel activation curve was unaffected. 6. From these data it is suggested that inhibition of mitochondrial, but not glycolytic, ATP production has stimulatory and inhibitory effects on voltage-gated Ca2+ channels of basilar artery smooth muscle cells. Effects of intracellular Mg2+ on the Ca2+ channels were modulated by ATP and mimicked the effects of metabolic poisoning by DNP. A hypothesis is discussed in which the intracellular free Mg2+ concentration may be a key factor coupling ATP production to Ca2+ channels.

Protein kinase A inhibitors selectively inhibit the tonic contraction of the guinea pig ureter to high potassium

1. We have investigated the effect of various protein kinase A (PKA) inhibitors on the phasic and tonic components of the response to potassium chloride (KCl) in the guinea pig ureter. All experiments were performed in ureters pretreated with capsaicin (10 microM for 15 min) to prevent the release of sensory neuropeptides and in the presence of 1 microM Bay K 8644 to maximize calcium (Ca) entry via voltage-sensitive channels. The addition of 80 mM hypertonic KCl produced maximal shortening of the ureter with distinct phasic and tonic components, the latter further showing a transient and a sustained component. Nifedipine (30 microM for 120 min) totally abolished all the responses to KCl. 2. The selective PKA inhibitor, H89 (10 microM), abolished the tonic response to KCl in about 30 min with minor inhibitory effect on the phasic contraction. This pattern was unchanged when extending the contact time to 120 min. When added 30 min before the next challenge, H89 (1-30 microM) concentration-dependently inhibited the responses to KCl with a preferential inhibitory effect on the tonic contraction. Another PKA inhibitor, H8, produced similar effects at tenfold higher concentrations (10-300 microM) than H89, consistent with the known potency ratio of these isoquinoline derivatives in inhibiting PKA. 3. The potent and nonselective protein kinase inhibitor, staurosporine (10-100 nM) produced an even depression of the various phases of the response to KCl. The selective protein kinase G inhibitor, KT 5823 (10 microM for 60 min) produced only a slight reduction of the sustained tonic response to KCl. The selective protein kinase C inhibitor GF 109,203X (1-3 microM) and the cAMP analog, Rp-cAMPS (300 microM for 60 min) had no effect on the three components of the response to KCl. 4. In the presence of Bay K 8644, electrical field stimulation (10 Hz for 1 sec, 60 V, pulse width 5 ms) produces direct myogenic phasic contractions (twitches) of the ureter which are suppressed by nifedipine (10-30 microM). H8 (up to 30 microM) and H89 (up to 300 microM) had minor effect on the amplitude of twitches, consistent with their poor inhibitory activity on the phasic responses to KCl. 5. In sucrose gap, superfusion with 80 mM hypertonic KCl produced action potentials followed by a sustained depolarization of the membrane: the two electrical responses underlie the phasic and tonic components of contraction to KCl, respectively. H89 (10 microM for 30 min) did not affect the resting membrane potential nor the KCl-evoked action potentials and sustained depolarization. H89 had no effect on the phasic contraction to KCl but markedly depressed (about 65% inhibition) the tonic contraction. 6. The present findings are consistent with the view that phosphorylation by PKA increases the availability of L-type Ca channels in the ureter smooth muscle. Blockade of PKA dissociates the electromechanical coupling between the sustained membrane depolarization produced by KCl and the corresponding sustained increase in tension. The L-type Ca channel responsible for generating action potentials and phasic contractions to KCl are less sensitive to PKA inhibitors than those responsible for the tonic contraction.

beta-Adrenoceptor stimulation activates large-conductance Ca2+-activated K+ channels in smooth muscle cells from basilar artery of guinea pig

We studied the effect of isoproterenol on the Ca2+-activated K+ (BK) channel in smooth muscle cells isolated from the basilar artery of the guinea pig. Cells were studied in a whole-cell configuration to allow the clamping of intracellular Ca2+ concentration, [Ca2+]i. Macroscopic BK channel currents were recorded during depolarizing test pulses from a holding potential (VH) of 0 mV, which was used to inactivate the outward rectifier. The outward macroscopic current available from a VH of 0 mV was highly sensitive to block by external tetraethylammonium Cl (TEA) and charybdotoxin, and was greatly augmented by increasing [Ca2+]i from 0.01 to 1.0 microM. With [Ca2+]i between 0.1 and 1.0 microM, 0.4 microM isoproterenol increased this current by 58.6 +/- 17.1%, whereas with [Ca2+]i at 0.01 microM a sixfold smaller increase was observed. With [Ca2+]i > or = 0.1 microM, 100 microM dibutyryl -adenosine 3':5: cyclic monophosphate (cAMP) and 1 microM forskolin increased this current by 58.5 +/- 24.1% and 59.7 +/- 10.3%, respectively. The increase with isoproterenol was blocked by 4.0 microM propranolol extracellularly, and by 10 U/ml protein kinase inhibitor intracellularly. Single-channel openings during depolarizing test pulses from a VH of 0 mV recorded in the whole-cell configuration under the same conditions (outside-out-whole-cell recording) indicated a slope conductance of 260 pS. In conventional outside-out patches, this 260-pS channel was highly sensitive to block by external TEA, and in inside-out patches, its probability of opening was greatly augmented by increasing [Ca2+]i from 0.01 to 1.0 microM. Outside-out-whole-cell recordings with [Ca2+]i > or = 0.1 microM indicated that 100 microM dibutyryl-cAMP increased the probability of opening of the 260-pS channel by 152 +/- 115%. In inside-out patches, the catalytic subunit of protein kinase A increased the probability of opening, and this effect also depended on [Ca2+]i , with a 35-fold larger effect observed with 0.1-0.5 microM Ca2+ compared to 0.01 microM Ca2+. We conclude that the BK channel in cerebrovascular smooth muscle cells can be activated by beta-adrenoceptor stimulation, that the effect depends strongly on [Ca2+]i, and that the effect is mediated by cAMP-dependent protein kinase A with no important contribution from a direct G-protein or phosphorylation-independent mechanism. Our data indicate that the BK channel may participate in beta-adrenoceptor-mediated relaxation of cerebral vessels, although the importance of this pathway in obtaining vasorelaxation remains to be determined.

Acetyl-L-carnitine arginyl amide (ST857) increases calcium channel density in rat pheochromocytoma (PC12) cells

We used the patch clamp technique to study the effect of acetyl-L-carnitine arginyl amide (ALCAA) and of nerve growth factor (NGF) on availability of L-type Ca2+ channels in rat pheochromocytoma (PC12) cells maintained in defined medium. Channel availability was measured as number of channels in the patch x the probability of opening (n.Po). In patches from control cells, cells exposed to NGF (10 ng/ml) for six days, and cells exposed to ALCAA (1 mM) for six days, n.Po, measured during 200-240 ms pulses to -10 mV (holding potential, -60 mV), was 0.102 +/- 0.089 (5 cells), 0.173 +/- 0.083 (5 cells), and 0.443 +/- 0.261 (7 cells), respectively. The 4.3-fold increase for the ALCAA-treated cells was significantly different from control (P < 0.05), whereas that for the NGF-treated cells was not. For the same conditions, the maximum number of superimposed openings at -10 mV was 1.3 +/- 0.5 (6 cells), 1.6 +/- 0.5 (8 cells), and 3.3 +/- 1.8 (8 cells), with the value for the ALCAA-treated cells being significantly different from control (P < 0.001). Additional analysis showed that the distribution of channel open times, the time constants, and the voltage dependence of activation were not changed by prolonged exposure to ALCAA. Short-term exposure to both ALCAA as well as to the parent compound, acetyl-L-carnitine (ALCAR), did not cause an increase but rather a decrease in n.Po, and this short-term effect of both compounds was blocked by neomycin, an inhibitor of phospholipase C.(ABSTRACT TRUNCATED AT 250 WORDS)

The patch clamp technique

The introduction of the patch clamp technique less than two decades ago revolutionized the study of cellular physiology by providing a high-resolution method of observing the function of individual ionic channels in a variety of normal and pathological cell types. By the use of variations of the basic recording methodology, cellular function and regulation can be studied at a molecular level by observing currents through individual ionic channels. At a cellular level, processes such as signaling, secretion, and synaptic transmission can be examined. In addition, by combining the information from high-resolution electrophysiological recordings obtained by the patch clamp method with modern molecular biological techniques, further insight can be gained into the gene expression and protein structure of ionic channels. Given the ubiquity and importance of ionic channels, it is not surprising that their study has led to a new understanding of the mechanisms of certain disease processes and has given insight into treatments for these diseases. This review gives an historical perspective of the development of the patch clamp technique and an overview of the methodologies currently in use. Examples are shown to illustrate typical uses of the patch clamp technique with emphasis on the variety of recording configurations available and the advantages and drawbacks of each method.