Calcium transport by sarcoplasmic reticulum of vascular smooth muscle: I. MgATP-dependent and MgATP-independent calcium uptake

Regulation of membrane excitability by intracellular pH (pHi) changers through Ca2+-activated K+ current (BK channel) in single smooth muscle cells from rabbit basilar artery

Employing microfluorometric system and patch clamp technique in rabbit basilar arterial myocytes, regulation mechanisms of vascular excitability were investigated by applying intracellular pH (pH(i)) changers such as sodium acetate (SA) and NH(4)Cl. Applications of caffeine produced transient phasic contractions in a reversible manner. These caffeine-induced contractions were significantly enhanced by SA and suppressed by NH(4)Cl. Intracellular Ca(2+) concentration ([Ca(2+)](i)) was monitored in a single isolated myocyte and based the ratio of fluorescence using Fura-2 AM (R (340/380)). SA (20 mM) increased and NH(4)Cl (20 mM) decreased R (340/380) by 0.2 +/- 0.03 and 0.1 +/- 0.02, respectively, in a reversible manner. Caffeine (10 mM) transiently increased R (340/380) by 0.9 +/- 0.07, and the ratio increment was significantly enhanced by SA and suppressed by NH(4)Cl, implying that SA and NH(4)Cl may affect [Ca(2+)](i) (p < 0.05). Accordingly, we studied the effects of SA and NH(4)Cl on Ca(2+)-activated K(+) current (IK(Ca)) under patch clamp technique. Caffeine produced transient outward current at holding potential (V (h)) of 0 mV, caffeine induced transient outward K(+) current, and the spontaneous transient outward currents were significantly enhanced by SA and suppressed by NH(4)Cl. In addition, IK(Ca) was significantly increased by acidotic condition when pH(i) was lowered by altering the NH(4)Cl gradient across the cell membrane. Finally, the effects of SA and NH(4)Cl on the membrane excitability and basal tension were studied: Under current clamp mode, resting membrane potential (RMP) was -28 +/- 2.3 mV in a single cell level and was depolarized by 13 +/- 2.4 mV with 2 mM tetraethylammonium (TEA). SA hyperpolarized and NH(4)Cl depolarized RMP by 10 +/- 1.9 and 16 +/- 4.7 mV, respectively. SA-induced hyperpolarization and relaxation of basal tension was significantly inhibited by TEA. These results suggest that SA and NH(4)Cl might regulate vascular tone by altering membrane excitability through modulation of [Ca(2+)](i) and Ca(2+)-activated K channels in rabbit basilar artery.

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.

The effects of extracellular and intracellular pH on intracellular Ca2+ regulation in guinea-pig detrusor smooth muscle

1. Intracellular pH (pHi) and intracellular [Ca2+] ([Ca2+]i) were measured during changes to superfusate PCO2 and/or [NaHCO3]. Changes to superfusate PCO2 produced sustained changes to pHi and [Ca2+]i, while changes to [NaHCO3] altered only extracellular pH (pHo). 2. Carbachol or caffeine induced a transient rise of [Ca2+]i due to Ca2+ release from an intracellular store. This Ca2+ transient was reduced by extracellular acidosis, but increased by intracellular acidosis. Alkalosis in either compartment produced opposite effects to acidosis. Changes to the Ca2+ transient mirrored those to phasic tension previously reported in this preparation. 3. A raised superfusate [K+] also induced a Ca2+ transient, due to transmembrane influx of Ca2+. This transient was depressed by extracellular acidosis, but unaffected by changes to pHi. The L-type Ca2+ current was similarly affected by changes to pHo, but not by alteration of pHi. 4. The results suggest that extracellular acidosis depresses the Ca2+ transient by reducing transmembrane influx through the L-type Ca2+ channel. The increase in the carbachol- and caffeine-induced Ca2+ transients by intracellular acidosis is due to enhancement of Ca2+ uptake into intracellular stores as a result of a raised resting [Ca2+]i.

Mechanisms for Ca signaling in vascular smooth muscle: resolved from 45Ca uptake and efflux experiments

Established cell lines are now widely used in experiments concerning vascular smooth muscle (VSM) function; however, considerable evidence suggests that cultured VSM cells are functionally different from VSM cells in intact blood vessels. In order to test the hypothesis that calcium signaling mechanisms are comparable in these two preparations, we developed a new method for high resolution 45Ca efflux studies in A7r5 cells. Briefly, this method involves plating cells in the lumen of a tubular glass efflux chamber and, after loading the cells with 45Ca, perfusing the chamber with a physiological saline solution and collecting the effluent. Using this method we found that the plasma membrane in cultured cells is not rate limiting for calcium efflux, since the efflux curves from both permeabilized and intact cells are kinetically the same. We also found the plasma membrane is not rate limiting in whole aortic segments by using a depolarizing solution followed by dihydropyridine solution. Thus, we demonstrated that the data obtained from cells or tissues with intact membranes reveal information about the intracellular stores (sarcoplasmic reticulum and mitochondria). Combining efflux data with a detailed kinetic model of cellular Ca transport allows least-squares estimation of the rate constants for release and uptake of Ca2+ by intracellular stores with a high degree of confidence (CV < 25%) as well as the Ca2+ contents and transmembrane fluxes associated with these stores. Quantitative comparison of results obtained from A7r5 cells with those we previously obtained for rabbit aortic segments reveals marked similarities and suggests that A7r5 cells serve as excellent model experiments for VSM cell Ca2+ homeostasis.

Properties of the sarcoplasmic reticulum Ca(2+)-pump in coronary artery skinned smooth muscle

Pig coronary artery cultured smooth muscle cells were skinned using saponin. In the presence of an ATP-regenerating system and oxalate, the skinned cells showed an ATP-dependent azide insensitive Ca(2+)-uptake which increased linearly with time for > 1 h. The Ca(2+)-uptake occurred with Km values of 0.20 +/- 0.03 microM for Ca2+ and 400 +/- 34 microM for MgATP2-. Thapsigargin and cyclopiazonic acid inhibited this uptake with IC50 values of 0.13 +/- 0.02 and 0.56 +/- 0.04 microM, respectively. These properties of SR Ca(2+)-pump are similar to those reported for membrane fractions isolated from fresh smooth muscle of coronary artery and other arteries. However, optimum pH of the uptake in the skinned cells (6.2) was lower than that reported previously using isolated membranes (6.4-6.8).

Effects of high-energy phosphates on carbachol-evoked cationic current in single smooth muscle cells from guinea-pig ileum

1. Single smooth muscle cells from the longitudinal muscle layer of guinea-pig small intestine were voltage clamped in the whole-cell recording mode with patch pipettes. The cationic current (Icat) evoked by application of 50 microM carbachol (CCh) was examined when free internal calcium in the cell was 'clamped' at 10(-7) M with 20 mM BAPTA. The effects of varying the composition of the pipette solution were studied. 2. Phosphocreatine (PCr, 6 mM) added to the pipette solution increased Icat by about 7-fold (to near 620 pA); lower concentrations had similar, generally lesser, effects. Na2ATP (3 or 6 mM) with or without 5 mM MgCl2 was much less effective than phosphocreatine alone. Addition of 3 mM Na2ATP reduced Icat, whether or not phosphocreatine was present. 3. Creatine (6 mM) with or without 2 mM Na2ATP was less effective than phosphocreatine in maintaining Icat. 4. GTP (0.1 mM) did not affect Icat evoked by CCh, whether phosphocreatine was present or not. 5. GTP gamma S (0.2 mM) included in pipette solution mimicked the effect of CCH and evoked Icat independently of whether PCr was present or not in the pipette solution. Including 5 mM ATP in the pipette reduced this current, whereas 5'-adenylyl imidodiphosphate (AMP-PNP) and ADP were without effect. 6. The results show that phosphocreatine increases membrane channel responsiveness to receptor activation and that ATP above 2 mM suppresses it.

Changes of intracellular [Ca2+] during refilling of sarcoplasmic reticulum in rat ventricular and vascular smooth muscle

1. Intracellular calcium concentration ([Ca2+]i) was measured in single myocytes isolated from either the cardiac ventricle or the mesenteric artery of the rat. 2. In both cardiac and smooth muscle, the application of caffeine produced an increase of [Ca2+]i which spontaneously decayed back to resting levels. In vascular smooth muscle cells, removal of caffeine produced a transient fall of [Ca2+]i to below the resting level. [Ca2+]i then returned to control levels. A transient undershoot of [Ca2+]i on removal of caffeine was also sometimes seen in cardiac cells. When the undershoot was absent in cardiac cells it could be induced by elevating [Ca2+]o. 3. In vascular smooth muscle cells noradrenaline increased [Ca2+]i and an undershoot of [Ca2+]i could be produced by its removal. In cardiac cells a small undershoot could sometimes be seen following the systolic Ca2+ transient produced by electrical stimulation. 4. In both cardiac and vascular cells the time constant of decay of the caffeine response (tau caff) was less than that of the recovery from the undershoot (tau us). On average the ratio tau us:tau caff was about 5 in smooth muscle. In cardiac cells the recovery of the undershoot was also considerably slower than that of the caffeine response. 5. If caffeine was removed before the rise of [Ca2+]i had fully decayed spontaneously then the magnitude of the undershoot was reduced. 6. It is suggested that the undershoot of [Ca2+]i on removal of caffeine results from refilling of the SR decreasing [Ca2+]i. The data from vascular cells can be fitted by this model if the dissociation constant, Kd, of the surface membrane Ca2+ pump for [Ca2+]i is about 1 microM. 7. Using the model, it is concluded from the ratio of the time constants shown above that the caffeine releasable content of the sarcoplasmic reticulum constitutes about 80% of total cellular calcium in both cardiac and smooth muscle.

Calcium transport by sarcoplasmic reticulum of vascular smooth muscle: II. Effects of calmodulin and calmodulin inhibitors

The role of calmodulin (CaM) in modulating calcium (Ca) uptake by sarcoplasmic reticulum (SR) of vascular smooth muscle was studied in saponin skinned strips of rat caudal artery. Exogenous CaM concentrations ranging from 0.3-1.8 microM did not statistically change the steady state MgATP-dependent Ca content, the MgATP-independent Ca content, or the oxalate-stimulated Ca influx. Calmidazolium (CDZ), W-7, and trifluoperazine (TFP) were used to examine the potential effect of an endogenous CaM pool on inward Ca transport. The IC50 of these antagonists for inhibition of Ca-CaM-stimulated phosphodiesterase activity and Ca-activated superprecipitation of canine aortic actomyosin was measured and found to be in the low micromolar range with a rank order of potency for inhibition of CDZ greater than TFP greater than W-7. In skinned tissues, micromolar concentrations of antagonists that inhibited CaM-mediated reactions in isolated enzyme systems did not reduce Ca content or oxalate-stimulated Ca influx. At higher concentrations of 100-200 microM, the MgATP-dependent Ca content was significantly reduced by TFP and W-7 but not by CDZ. The order of potency for inhibition of Ca uptake was TFP greater than W-7 greater than CDZ. The MgATP-independent Ca content was significantly decreased only by 200 microM TFP. Although none of these inhibitors significantly altered Ca efflux at concentrations up to 100 microM, Ca release was significantly stimulated by all three at 200 microM. The TFP-stimulated Ca release was partially inhibited by ruthenium red. The results indicate that neither exogenous CaM nor an endogenous CaM pool directly modulates inward Ca transport by the SR of saponin skinned caudal artery. The inhibition of Ca uptake produced by hundred micromolar concentrations of CaM antagonists fails to correlate with the order of and with the potency of inhibition measured in isolated enzyme systems. This suggests that the inhibition of Ca uptake produced by high concentrations of these antagonists may be independent of a specific interaction with CaM. The activation of Ca release by high concentrations of CaM antagonists may involve a nonspecific increase in membrane permeability as well as modulation of a membrane Ca channel.