内皮素对培养心肌细胞内游离钙浓度的作用

实验用培养新生ＳＤ大鼠心室肌细胞,以Ｆｕｒａ－２／ＡＭ荧光指示剂负载检测收肌细胞内游离钙浓度（「Ｃａ＾２＋」）的变化,探讨内皮素－１（ＥＴ－１）对「Ｃａ＾２＋」ｉ的作用及其机制。结果显示：ＥＴ－１引起心肌细胞「Ｃａ＾２＋」ｉ升高有两个时相,瞬时相持续相。ＥＴ－１诱导的瞬时相「Ｃａ＾２＋」ｉ升高呈浓度依赖性,预先用ＥＴＡ特异性受阻断剂ＢＱ１２３处理,可阻断ＥＴ－１引起的「Ｃａ＾２＋」ｉ升高,揭示上述     

Effects of Cl- channel blockers on endothelin-1-induced proliferation of rat vascular smooth muscle cells

The effects of Cl- channel blockers on endothelin-1 (ET-1)-induced proliferation of rat aortic vascular smooth muscle cells (VSMC) were examined. We found ET-1 concentration-dependently increased cell count and [3H]-thymidine incorporation into VSMC, with EC50 values of 24.8 and 11.4 nM, respectively. Both nifedipine and SK&F96365 inhibited 10 nM ET-1-induced [3H]-thymidine incorporation into VSMC with the maximal inhibitory concentrations of 1 and 10 microM, respectively. DIDS inhibited 10 nM ET-1-induced increase in cell count and [3H]-thymidine incorporation into VSMC in a concentration-dependent manner, whereas other Cl- channel blockers including IAA-94, NPPB, DPC, SITS and furosemide did not produce these effects. 3 microM DIDS reduced 10 nM ET-1-induced sustained increase in cytoplasmic Ca2+ concentration ([Ca2+]) by 52%. Pretreatment of VSMC with 1 microM nifedipine completely inhibited the DIDS effect on 10 nM ET-1-induced [3H]-thymidine incorporation into VSMC and sustained increase in [Ca2+]i, whereas pretreatment with 10 microM SK&F96365 did not completely block these effects of DIDS. DIDS did not affect ET-1-induced Ca2+ release and 30 mM KCl-induced increase in [Ca2+]i. Our data suggest that DIDS-sensitive Cl- channels mediate VSMC proliferation induced by ET-1 by mechanisms related to membrane depolarization and Ca2+ influx through voltage-dependent Ca2+ channels.     

Regulation of the plasma membrane Ca2+ pump by cyclic nucleotides in cultured vascular smooth muscle cells

We investigated the mechanisms of Ca2+ extrusion from cultured rat aortic smooth muscle cells while monitoring changes in the cytosolic Ca2+ concentration ([Ca2+]i) using fura 2 fluorescence. 45Ca2+ efflux from these cells consisted of two major mechanisms; one was dependent on the extracellular sodium concentration (Na+o) and the other was independent of Na+o. Na+o-dependent efflux increased monotonically with increasing [Ca2+]i between 0.1 and 1.0 microM, whereas Na+o-independent efflux reached a plateau at 0.6-1 microM [Ca2+]i with a half-maximum obtained at about 0.16 microM. At [Ca2+]i below 1 microM, the latter was significantly greater than the former. Unlike the Na+o-dependent mechanism, Na+o-independent 45Ca2+ efflux was inhibited almost entirely by extracellularly added La3+ or a combination of high extracellular pH (pH 8.8) and 20 mM Mg2+. It was also inhibited, although not completely, by compound 48/80, a calmodulin antagonist, and vanadate. These results strongly suggest that Na+o-dependent and Na+o-independent 45Ca2+ effluxes occur via the Na+/Ca2+ exchanger and the ATP-dependent Ca2+ pump, respectively. Sodium nitroprusside and atrial natriuretic factor, which are agents that stimulate intracellular production of cGMP, and 8-BrcGMP significantly accelerated the Na+o-independent 45Ca2+ efflux especially at low [Ca2+]i. Forskolin, dibutyryl cAMP, and 8-Br-cAMP, however, showed no stimulation. These results suggest that the plasma membrane Ca2+ pump is regulated by cGMP but not by cAMP in intact vascular smooth muscle cells.     

Production of nitric oxide from endothelial cells by 31-amino-acid-length endothelin-1, a novel vasoconstrictive product by human chymase

Human chymase selectively converts big endothelin (ET)-1 to 31-amino-acid-length ET-1 [ET-1(1-31)]. In this study we examined effect of ET-1(1-31) on endothelial function. ET-1(1-31) evoked contraction in a concentration-dependent manner at > 10(-8) M, which was about 10 times weaker than that of conventional ET-1 [ET-1(1-21)]. BQ485, an ETA receptor antagonist, completely abolished ET-1(1-31)-induced contraction, but BQ788, an ETB receptor antagonist, slightly enhanced it, suggesting that ET-1(1-31) relaxes artery via endothelium. On endothelial cells, ET-1(1-21) and ET-1(1-31) increased [Ca2+]i and produced NO, both of which were significantly inhibited by BQ788 and not by BQ485. These results indicate that ET-1(1-31) increased [Ca2+]i and produced NO in endothelial cells through ETB receptor similarly with ET-1(1-21), although slight difference in effect on smooth muscle cells.     

Downregulation of caveolin expression by cAMP signal

Recent data have demonstrated that caveolin, a major structural protein of caveolae, inhibits the function of molecules involved in cAMP signaling such as adenylyl cyclase. We examined the effect of cAMP signal on the expressions of caveolin subtypes using rat cardiac myoblasts (H9C2 cells) and smooth muscle cells (RASMC), which express caveolin subtypes. Treatment of RASMC and H9C2 cells with forskolin, an adenylyl cyclase stimulator, decreased caveolin-1 mRNA levels in a dose-dependent manner. Time course studies showed a time-dependent decrease of caveolin-1 mRNA levels in H9C2 cells (after 6 hours) while caveolin-1 mRNA levels in RASMC showed a biphasic response, i.e., an initial increase (within 3 hours) and a later decrease (after 3 hours). Similar biphasic changes were observed when RASMC was treated with IBMX, a phosphodiesterase inhibitor. The levels of caveolin-1 and -3 proteins were also decreased by forskolin treatment, but only after 60-72 hours in RASMC and 24-36 hours in H9C2 cells. In contrast, the expression of caveolin-2 remained similar in both cells and decreased to a small degree after prolonged treatment. Therefore, the expression of caveolin is downregulated by cAMP signal in a caveolin subtype-dependent manner.     

17β—雌二醇下调血管平滑肌内皮素A型受体的表达

为进一步探讨雌激素对心血管的保护作用,实验在双侧卵巢去势大鼠模型和培养的血管平滑肌细胞（VSMCs)上,观察17β－雌二醇（E2）对血管反应性及VSMCs增殖的影响,以RT－PCR和Western blot检测内皮素受体（ETAR）的表达,结果显示：去势雌性大鼠血管对内皮素（ET－1）的反应性明显增高,ETAR特异性受体阻断剂BQ123能完全阻断ET－1对VSMCs增殖的影响,E2能明显抑制ET－1对VSMCs增殖的作用,RT－PCR结果显示E2能抑制ETAR mRNA的表达,Western blot进一步证实E2能抑制ETAR蛋白表达,E2受体阻断剂Tamoxifen能部分抑制ET－1对VSMCs的增殖及ETAR的mRNA和蛋白 的表达。以上结果提示;ET－1促VSMCs增殖的效应主要是由ETAR介导的,雌激素能通过下调ETAR来抑制ET－1对VSMCs 促增殖的作用和血管对ET－1的反应,且此作用与雌激素受体有关。     

Direct regulation of smooth muscle contractile elements by second messengers

The effects of adenosine 3',5'-cyclic monophosphate (cAMP), guanosine 3',5'-cyclic monophosphate (cGMP) and phorbol 12,13 dibutyrate (PDBu) on the Ca2+ sensitivity of the contractile elements in the rat mesenteric artery were investigated, using a method of permeabilizing smooth muscle with Staphylococcal alpha-toxin. Both cAMP and cGMP relaxed the permeabilized rat mesenteric artery at the intracellular Ca2+ concentrations [( Ca2+]i) held constant with Ca2+ EGTA buffer and Ca2+ ionophore, ionomycin. In addition, forskolin and sodium nitroprusside which activate adenylate and guanylate cyclases, respectively, also induced relaxation at a fixed [Ca2+]i. In contrast PDBu which stimulates protein kinase C caused an increase in force at a constant [Ca2+]i which could be partially reversed by cAMP or cGMP. These results indicate that second messengers exert direct control over smooth muscle Ca2+ sensitivity of the contractile elements, which is of physiologic and pharmacologic importance.     

Genistein inhibits contractile force, intracellular Ca2+ increase and Ca2+ oscillations induced by serotonin in rat aortic smooth muscle

The soy-derived isoflavones genistein and daidzein affect the contractile state of different kinds of smooth muscle. We describe acute effects of genistein and daidzein on contractile force and intracellular Ca2+ concentration ([Ca2+]i) in in situ smooth muscle of rat aorta. Serotonin (5-HT) (2 microM) or a depolarizing high K+ solution produced the contraction of aortic rings, which were immediately relaxed by 20 microM genistein and by 20 microM daidzein. Accordingly, both 5-HT and a high K+ solution increased the [Ca2+]i in in situ smooth muscle cells. Genistein strongly inhibited the [Ca2+]i increase evoked by 5-HT (74.0 +/- 7.3%, n = 11, p < 0.05), and had a smaller effect on high K+ induced [Ca2+]i increase (19.9 +/- 4.0%, n = 7, p < 0.05). The K+ channels blocker tetraethylammonium (TEA) (0.5 mM) diminished genistein effects on 5-HT-induced [Ca2+]i increase. Interestingly, during prolonged application of 5-HT, the [Ca2+]i oscillated and a short (90 s) preincubation with genistein (20 microM) significantly diminished the frequency of the oscillations. This effect was totally abolished by TEA. In conclusion, in rat aortic smooth muscle, genistein is capable of diminishing the increase in [Ca2+]i and in force evoked by 5-HT and high K+ solution, and of decreasing the frequency of [Ca2+]i oscillations induced by 5-HT. The short time required by genistein, and the relaxing effect of daidzein suggest that tyrosine kinases inhibition is not involved. The small inhibiting effect of genistein on the [Ca2+]i increase evoked by high K+ and the effect of TEA point to the activation by genistein of calcium-activated K+ channels.     

Differential regulation of intracellular Ca2+ signalling induced by high K+ and endothelin-1 in single smooth muscle cells of intact canine basilar artery: detection by means of confocal laser microscopy

Changes in intracellular calcium concentration ([Ca2+]i) in smooth muscle cells play the key role in regulation of vascular smooth muscle tone and pathogenesis of cerebral vasospasm. In this study, we adopted the confocal laser microscopy to detect the fluorescence signals arising from the individual smooth muscle cells of canine basilar artery. Ring preparations were made, loaded with fluo-3 and changes in fluorescence induced by high K+ and endothelin-1 (ET-1) were measured by confocal laser microscopy. In some unstimulated smooth muscle cells Ca2+ waves arising from discrete region of the cell propagated to the whole cell with a velocity of approximately 10 microm/s. High K+ (80 mmol/L) induced a rapid rise in [Ca2+]i, the peak level being consistently reached approximately 10 s after stimulation. In contrast, the time to peak level of [Ca2+]i induced by ET-1 (0.3 micromol/L) varied widely between 13 and 26 s among individual cells, an indication that the extent of nonuniform coordination of increases in [Ca2+]i in individual cells may be partly responsible for the different time courses of tension development of vascular smooth muscle in response to the vasoactive stimulants. The increase in [Ca2+]i induced by ET-1 was transient but a pronounced and sustained contraction developed further in response to ET-1. Thus ET-1 has a biological property as a potential candidate to elicit cerebral vasospasm. Confocal laser microscopy could be a useful tool to measure the changes in [Ca2+]i in individual smooth muscle cells of cerebral artery.     

Biological profiles of highly potent novel endothelin antagonists selective for the ETA receptor.

We describe novel potent endothelin (ET) antagonists that are highly potent and selective for the ETA receptor (selective to ET-1). Of the synthetic analogs based on ETA antagonist BE-18257A isolated from Streptomyces misakiensis (IC50 value for ETA receptor on porcine aortic smooth muscle cells (VSMCs); 1.4 microM), the compounds BQ-123 and BQ-153 greatly improved the binding affinity of [125I]ET-1 for ETA receptors on VSMCs (IC50; 7.3 and 8.6 nM, respectively), whereas they barely inhibited [125I]ET-1 binding to ETB receptors (nonselective with respect to isopeptides of ET family) in the cerebellar membranes (IC50; 18 and 54 microM, respectively). Associated with the increased affinity for ETA receptors, these peptides antagonized ET-1-induced constriction of isolated porcine coronary artery. However, there was a small amount of ET-1-induced vasoconstriction resistant to these antagonists, which paralleled the incomplete inhibition of [125I]ET-1 binding in the membrane of the aortic smooth muscle layer. These data suggest that the artery has both ETA and ETB receptors responsible for ET-1-induced vasoconstriction. The antagonists shifted the concentration-response curve to the right for ET-1 in the coronary artery, and increased the apparent dissociation constant in the Scatchard analysis of [125I]ET-1 binding on the VSMCs without affecting the binding capacity, indicative of the competitive antagonism for ETA receptor. In conscious rats, pretreatment with the antagonists markedly antagonized ET-1-induced sustained pressor responses in dose-dependent fashion without affecting ET-1-induced transient depressor action, suggesting that the pressor action is mediated by ETA receptors, while the depressor action is mediated by ETB receptors. In addition, pretreatment with the potent antagonists prevented ET-1-induced sudden death in mice. Thus, these potent ETA antagonists should provide a powerful tool for exploring the therapeutic uses of ETA antagonists in putative ET-1-related disorders.     

Production of NAADP and its role in Ca2+ mobilization associated with lysosomes in coronary arterial myocytes

The present study was designed to determine the production of nicotinic acid adenine dinucleotide phosphate (NAADP) and its role associated with lysosomes in mediating endothelin-1 (ET-1)-induced vasoconstriction in coronary arteries. HPLC assay showed that NAADP was produced in coronary arterial smooth muscle cells (CASMCs) via endogenous ADP-ribosyl cyclase. Fluorescence microscopic analysis of intracellular Ca2+ concentration ([Ca2+]i) in CASMCs revealed that exogenous 100 nM NAADP increased [Ca2+]i by 711 +/- 47 nM. Lipid bilayer experiments, however, demonstrated that NAADP did not directly activate ryanodine (Rya) receptor Ca2+ release channels on the sarcoplasmic reticulum. In CASMCs pretreated with 100 nM bafilomycin A1 (Baf), an inhibitor of lysosomal Ca2+ release and vacuolar proton pump function, NAADP-induced [Ca2+]i increase was significantly abolished. Moreover, ET-1 significantly increased NAADP formation in CASMCs and resulted in the rise of [Ca2+]i in these cells with a large increase in global Ca2+ level of 1,815 +/- 84 nM. Interestingly, before this large Ca2+ increase, a small Ca2+ spike with an increase in [Ca2+]i of 529 +/- 32 nM was observed. In the presence of Baf (100 nM), this ET-1-induced two-phase [Ca2+]i response was completely abolished, whereas Rya (50 microM) only markedly blocked the ET-1-induced large global Ca2+ increase. Functional studies showed that 100 nM Baf significantly attenuated ET-1-induced maximal constriction from 82.26 +/- 4.42% to 51.80 +/- 4.36%. Our results suggest that a lysosome-mediated Ca2+ regulatory mechanism via NAADP contributes to ET-1-induced Ca2+ mobilization in CASMCs and consequent vasoconstriction of coronary arteries.     

Fendiline-Induced Ca2+ movement in A10 smooth muscle cells

The effect of fendiline, an anti-anginal drug, on cytosolic free Ca2+ levels ([Ca2+]i) in A10 smooth muscle cells was explored by using fura-2 as a Ca2+ indicator. Fendiline at concentrations between 10-50 microM increased [Ca2+]i in a concentration-dependent manner with an EC50 of 20 microM. External Ca2+ removal reduced the Ca2+ signal by 75%. Addition of 3 mM Ca2+ increased [Ca2+]i in cells pretreated with fendiline in Ca2+-free medium. The 50 microM fendiline-induced [Ca2+]i increase in Ca2+-containing medium was inhibited by 10 microM of La3+, nifedipine, or verapamil. In Ca2+-free medium, pretreatment with 1 microM thapsigargin (an endoplasmic reticulum Ca2+ pump inhibitor) to deplete the endoplasmic reticulum Ca2+ store partly inhibited 50 microM fendiline-induced Ca2+ release; whereas pretreatment with 50 microM fendiline abolished 1 microM thapsigargin-induced Ca2+ release. Inhibition of phospholipase C activity with 2 microM U73122 did not alter 50 microM fendiline-induced Ca2+ release. Incubation with 50 microM fendiline for 10-30 min decreased cell viability by 10-20%. Together, the findings indicate that in smooth muscle cells fendiline induced [Ca2+]i increases. Fendiline acted by activating Ca2+ influx via L-type Ca2+ channels, and by releasing internal Ca2+ in a phospholipase C-independent manner. Prolonged exposure of cells to fendiline induced cell death.     

Time course of the initial [Ca2+]i response to extracellular ATP in smooth muscle depends on [Ca2+]e and ATP concentration.

In response to extracellular application of 50 microM ATP, all individual porcine aortic smooth muscle cells respond with rapid rises from basal [Ca2+]i to peak [Ca2+]i within 5 s. The time from stimulus to the peak of the [Ca2+]i response increases with decreasing concentration of ATP. At ATP concentrations of 0.5 microM and below, the time to the [Ca2+]i peak varies more significantly from cell to cell than at higher concentrations, and each cell shows complicated initiation and decay kinetics. For any individual cell, the lag phase before a response decreases with increasing concentration of ATP. An increase in lag time with decreasing ATP concentration is also observed in the absence of extracellular Ca2+, but the lag phase is more pronounced, especially at concentrations of ATP below 0.5 microM. Whole-cell patch-clamp electrophysiology shows that in porcine aortic smooth muscle cells, ATP stimulates an inward current carried mainly by Cl- ion efflux with a time course similar to the [Ca2+]i changes and no detectable current from an ATP-gated cation channel. A simple signal cascade initiation kinetics model, starting with nucleotide receptor activation leading to IP3-mediated Ca2+ release from IP3-sensitive internal stores, fits the data and suggests that the kinetics of the Ca2+ response are dominated by upstream signal cascade components.     

Role of calcium and cAMP in the action of adrenocorticotropin on aldosterone secretion

When the dose-response curve of adrenocorticotropin (ACTH)-induced aldosterone secretion is compared to that of ACTH-induced intracellular cAMP, the ED50 for intracellular cAMP is more than 10 times as high as that for aldosterone production. In contrast, the dose-response curve of forskolin-induced aldosterone secretion correlates well with that for forskolin-induced intracellular cAMP. ACTH, but not forskolin, increases calcium influx into glomerulosa cells without inducing the mobilization of calcium from an intracellular pool. The effect of ACTH on calcium influx is dose-dependent and ED50 is 3.5 X 10(-11) M. In a perifusion system, the effect of 1 nM ACTH on aldosterone secretion is much greater than that of 1 microM forskolin, even though these two stimulators induce identical increases in the intracellular cAMP. Perifusion with combined A23187 (50 nM) and forskolin (1 microM) stimulates aldosterone secretion to a value comparable to that induced by 1 nM ACTH. Likewise, BAY K 8644 (1 nM), which induces a comparable increase in calcium influx, potentiates the effect of 1 microM forskolin. When the intracellular [Ca2+] is fixed at either 100 or 300 nM, forskolin-stimulated intracellular cAMP content is identical, but ACTH-stimulated intracellular cAMP content at 100 nM [Ca2+]i is 60% of that at 300 nM [Ca2+]i. Both the ACTH- and forskolin-induced aldosterone secretion rate is higher at 300 nM than at 100 nM [Ca2+]i. These results indicate that ACTH stimulates calcium influx, that calcium potentiates ACTH-induced but not forskolin-induced cAMP generation, and that Ca2+ and cAMP act as synarchic messengers in ACTH-mediated aldosterone secretion.     

Cyclic AMP inhibits secretion from bovine adrenal chromaffin cells evoked by carbamylcholine but not by high K+

The role of cAMP in the control of secretion from bovine adrenal chromaffin cells was examined using the adenylate cyclase activator, forskolin. Treatment of chromaffin cells with forskolin resulted in a rise in cAMP levels. Forskolin inhibited catecholamine release elicited by carbamylcholine or nicotine but had no effect on secretion evoked by 55 mM K+. Inhibition of carbamylcholine-stimulated release by forskolin was half-maximal at 10 microM forskolin. The inhibition by forskolin of secretion evoked by carbamylcholine was at a step distal to the rise in intracellular free calcium concentration ([Ca2+]i), since this rise was not inhibited by forskolin, which itself produced a small rise in [Ca2+]i. The results suggest that secretion evoked by carbamylcholine is due to the activation of an additional second messenger pathway acting with the rise in [Ca2+]i. This additional pathway may be the target for cAMP action.     

Role of M2 muscarinic receptors in airway smooth muscle contraction

Airway smooth muscle expresses both M2 and M3 muscarinic receptors with the majority of the receptors of the M2 subtype. Activation of M3 receptors, which couple to Gq, initiates contraction of airway smooth muscle while activation of M2 receptors, which couple to Gi, inhibits beta-adrenergic mediated relaxation. Increased sensitivity to intracellular Ca2+ is an important mechanism for agonist-induced contraction of airway smooth muscle but the signal transduction pathways involved are uncertain. We studied Ca2+ sensitization by acetylcholine (ACh) and endothelin-1 (ET-1) in porcine tracheal smooth muscle by measuring contractions at constant [Ca2+] in strips permeabilized with Staphylococcal alpha-toxin. Both ACh and ET-1 contracted airway smooth muscle at constant [Ca2+]. Pretreatment with pertussis toxin for 18-20 hours reduced ACh contractions, but had no effect on those of ET-1 or GTPgammaS. We conclude that the M2 muscarinic receptor contributes to airway smooth muscle contraction at constant [Ca2+] via the heterotrimeric G-protein Gi.     

Interaction between endothelium-derived relaxing factors, S-nitrosothiols, and endothelin-1 on Ca2+ mobilization in rat vascular smooth muscle cells.

S-Nitrosothiols (S-nitrosocysteine, S-nitrosoglutathione and S-nitroso-N-acetylpenicillamine), which belong to the group of endothelium-derived relaxing factors (EDRFs), caused decreases of cytosolic free Ca2+ concentrations ([Ca2+]i) in cultured rat vascular smooth muscle cells (VSMCs). The endothelin-1 (ET-1)-induced sustained increase of [Ca2+]i in rat VSMCs was completely abolished by preaddition of at least an equal molar quantity of S-nitrosocysteine (Cys-SNO). Also exposure of VSMCs to a mixture of Cys-SNO and ET-1 at the same time resulted in the transient increase only. These results suggest that S-nitrosothiols may have no significant effect on ET-1-induced Ca2+ release from intracellular stores via inositol 1,4,5-triphosphate production but do affect Ca2+ influx through Ca2+ channels in the plasma membrane.     

CGRP receptors in the gerbil spiral modiolar artery mediate a sustained vasodilation via a transient cAMP-mediated Ca2+-decrease

Alteration of cochlear blood flow may be involved in the etiology of inner ear disorders like sudden hearing loss, fluctuating hearing loss and tinnitus. The aim of the present study was to localize the vasodilator calcitonin gene-related peptide (CGRP) and to identify CGRP receptors and their signaling pathways in the gerbil spiral modiolar artery (SMA) that provides the main blood supply of the cochlea. CGRP was localized in perivascular nerves by immunocytochemistry. The vascular diameter and cytosolic Ca2+ concentration [Ca2+]i in the smooth muscle cells were measured simultaneously with videomicroscopy and fluo-4-microfluorometry. Calcitonin receptor-like receptor (CRLR) mRNA was identified by RT-PCR as a specific 288 bp fragment in total RNA isolated from the vascular wall. The SMA was preconstricted by a 2-min application of 1 nM endothelin-1 (ET1). CGRP, forskolin, and dibutyryl-cAMP caused a vasodilation (EC50 = 0.1 nM, 0.3 mM, and 20 mM). CGRP and forskolin caused an increase in cAMP production and a transient decrease in the [Ca2+]i. The CGRP-induced vasodilation was antagonized by CGRP8-37 (KDB = 2 mM). The K+-channel blockers iberiotoxin and glibenclamide partially prevented the CGRP- or forskolin-induced vasodilations but failed to reverse these vasodilations. These results demonstrate that CGRP is present in perivascular nerves and causes a vasodilation of the ET1-preconstricted SMA. The data suggest that this vasodilation is mediated by an increase in the cytosolic cAMP concentration, a transient activation of iberiotoxin-sensitive BK and glibenclamide-sensitive KATP K+ channels, a transient decrease in the [Ca2+]i and a long-lasting Ca2+ desensitization.