Handling of cytoplasmic Ca2+ by the sarcoplasmic reticulum during alpha 1-adrenoceptor-mediated contraction of rat mesenteric resistance arteries

Contribution of both Ca2+ entry and Ca2+ sensitization to the alpha1-adrenergic vasoconstriction of rat penile small arteries

Sympathetic adrenergic nerves maintain the flaccid state of the penis through the tonic release of norepinephrine that contracts trabecular and arterial smooth muscle. Simultaneous measurements of intracellular Ca(2+) concentration ([Ca(2+)](i)) and tension and experiments with alpha-toxin-permeabilized arteries were performed in branches of the rat dorsal penile artery to investigate the intracellular Ca(2+) signaling pathways underlying alpha(1)-adrenergic vasoconstriction. Phenylephrine increased both [Ca(2+)](i) and tension, these increases being abolished by extracellular Ca(2+) removal and reduced by about 50% by the L-type Ca(2+) channel blocker nifedipine (0.3 microM). Non-L-type Ca(2+) entry through store-operated channels was studied by inhibiting the sarcoplasmic reticulum Ca(2+)-ATPase with cyclopiazonic acid (CPA). CPA (30 microM) induced variable phasic contractions that were abolished by extracellular Ca(2+) removal and by the store-operated channels antagonist 2-aminoethoxydiphenyl borate (2-APB, 50 microM) and largely inhibited by nifedipine (0.3 microM). CPA induced a sustained increase in [Ca(2+)](i) that was reduced in a Ca(2+)-free medium. Under conditions of L-type channels blockade, Ca(2+) readmission after store depletion with CPA evoked a sustained and marked elevation in [Ca(2+)](i) not coupled to contraction. 2-APB (50 microM) inhibited the rise in [Ca(2+)](i) evoked by CPA and the nifedipine-insensitive increases in both [Ca(2+)](i) and contraction elicited by phenylephrine. In alpha-toxin-permeabilized penile arteries, activation of G proteins with guanosine 5'-O-(3-thiotriphosphate) and of the alpha(1)-adrenoceptor with phenylephrine both enhanced the myofilament sensitivity to Ca(2+). This Ca(2+) sensitization was reduced by selective inhibitors of PKC, tyrosine kinase (TK), and Rho kinase (RhoK) by 43%, 67%, and 82%, respectively. As a whole, the present data suggest the alpha(1)-adrenergic vasoconstriction in penile small arteries involves Ca(2+) entry through both L-type and 2-APB-sensitive receptor-operated channels, as well as Ca(2+) sensitization mechanisms mediated by PKC, TK, and RhoK. A capacitative Ca(2+) entry coupled to noncontractile functions of the smooth muscle cell is also demonstrated.

The sarcoplasmic reticulum and sarcolemma together form a passive Ca2+ trap in colonic smooth muscle

In smooth muscle, active Ca(2+) uptake into regions of sarcoplasmic reticulum (SR) which are closely apposed to the sarcolemma has been proposed to substantially limit the increase in the cytoplasmic Ca(2+) concentration ([Ca(2+)](c)) following Ca(2+) influx, i.e. the 'superficial buffer barrier hypothesis'. The present study has re-examined this proposal. The results suggest that the SR close to the sarcolemma acts as a passive barrier to Ca(2+) influx limiting [Ca(2+)](c) changes; for this, SR Ca(2+) pump activity is not required. In single voltage-clamped colonic myocytes, sustained opening of the ryanodine receptor (RyR) (and depletion of the SR) using ryanodine increased the amplitude of depolarisation-evoked Ca(2+) transients and accelerated the rate of [Ca(2+)](c) decline following depolarisation. These results could be explained by a reduction in the Ca(2+) buffer power of the cytosol taking place when RyR are opened (i.e. the SR is 'leaky'). Indeed, determination of the Ca(2+) buffer power confirmed it was reduced by approximately 40%. Inhibition of the SR Ca(2+) pump (with thapsigargin) also depleted the SR of Ca(2+) but did not reduce the Ca(2+) buffer power or increase depolarisation-evoked Ca(2+) transients and slowed (rather than accelerated) Ca(2+) removal. However, thapsigargin prevented the ryanodine-induced increase in [Ca(2+)](c) decline following depolarisation. Together, these results suggest that when the SR was rendered 'leaky' (a) more of the Ca(2+) entering the cell reached the bulk cytoplasm and (b) Ca(2+) was removed more quickly at the end of cell activation. Under physiological circumstances in the absence of blocking drugs, it is proposed that the SR limits the [Ca(2+)](c) increase following influx without the need for active Ca(2+) uptake. The SR and sarcolemma may form a passive physical barrier to Ca(2+) influx, a Ca(2+) trap, which limits the [Ca(2+)](c) rise occurring during depolarisation by about 50% and from which the ion only slowly escapes into the main part of the cytoplasm.

Capacitative calcium entry as a pulmonary specific vasoconstrictor mechanism in small muscular arteries of the rat

(1) The effect of induction of capacitative Ca2+ entry (CCE) upon tone in small (i.d. 200-500 microm) intrapulmonary (IPA), mesenteric (MA), renal (RA), femoral (FA), and coronary arteries (CA) of the rat was examined. (2) Following incubation of IPA with 100 nm thapsigargin (Thg) in Ca2+-free physiological salt solution (PSS), a sustained contraction was observed upon reintroduction of 1.8 mm Ca2+, which was unaffected by either diltiazem (10 microm) or the reverse mode Na+/Ca2+ antiport inhibitor KB-R7943 (10 microm). An identical protocol failed to elicit contraction in MA, RA, or CA, while a small transient contraction was sometimes observed in FA. (3) The effect of this protocol on the intracellular Ca2+ concentration ([Ca2+]i) was assessed using Fura PE3-loaded IPA, MA, and FA. Reintroduction of Ca2+ into the bath solution following Thg treatment in Ca2+-free PSS caused a large, rapid, and sustained increase in [Ca2+]i in all the three types of artery. (4) 100 nm Thg induced a slowly developing noisy inward current in smooth muscle cells (SMC) isolated from IPA, which was due to an increase in the activity of single channels with a conductance of approximately 30 pS. The current had a reversal potential near 0 mV in normal PSS, and persisted when Ca2+-dependent K+ and Cl- currents were blocked; it was greatly inhibited by 1 microm La3+, 1 microm Gd3+, and the IP3 receptor antagonist 2-APB (75 microm), and by replacement of extracellular cations by NMDG+. (5) In conclusion, depletion of intracellular Ca2+ stores with Thg caused capacitative Ca2+ entry in rat small muscular IPA, MA, and FA. However, a corresponding contraction was observed only in IPA. CCE in IPA was associated with the development of a small La3+- and Gd3+-sensitive current, and an increased Mn2+ quench of Fura PE-3 fluorescence. These results suggest that although CCE occurs in a number of types of small arteries, its coupling to contraction appears to be of particular importance in pulmonary arteries.

Ca(2+) uptake function of sarcoplasmic reticulum during contraction of rat arterial smooth muscles

To determine the Ca(2+) uptake function of the sarcoplasmic reticulum during contraction, the effects of cyclopiazonic acid or thapsigargin, agents that inhibit sarcoplasmic reticulum Ca(2+)-ATPase, on the contractile responses to K(+) or norepinephrine were compared in endothelium-denuded strips of femoral, mesenteric and carotid arteries of the rat. The addition of K(+) (3-20 mM) to the strips caused a concentration-dependent contraction, and the sensitivity to K(+) was much higher in the carotid artery than in the other arteries. The preincubation of strips with cyclopiazonic acid (10 microM) or thapsigargin (100 nM) caused a leftward shift of the concentration-response curve for K(+), and this effect was smaller in the carotid artery than in the other arteries. Inhibition of sarcoplasmic reticulum Ca(2+) uptake caused the sensitivity to K(+) to be similar in the three arteries. Similar results were obtained when the contractile responses to norepinephrine were determined. Cyclopiazonic acid itself induced similar transient contractions in the three arteries. The addition of caffeine (20 mM) caused a transient contraction that was smaller in the carotid artery than in the other arteries. We conclude that (1) the Ca(2+) influx during stimulation with K(+) or norepinephrine is buffered by the sarcoplasmic reticulum in femoral and mesenteric arteries, (2) this function is weak in the carotid artery, probably because the sarcoplasmic reticulum of this artery is almost filled with Ca(2+) in the resting state, and (3) the Ca(2+) uptake function of the sarcoplasmic reticulum during contraction is reflected by the contractile sensitivity in these arteries.

Ca(2+) movement from leaky sarcoplasmic reticulum during contraction of rat arterial smooth muscles

To examine the Ca(2+) buffering function of the sarcoplasmic reticulum during arterial contraction, we studied Ca(2+) movement during stimulation with K(+) or norepinephrine in arteries with a leaky sarcoplasmic reticulum. Responses were compared in endothelium-denuded strips of femoral, mesenteric and carotid arteries of the rat. To make the sarcoplasmic reticulum leaky to Ca(2+), Ca(2+)-induced Ca(2+) release channels of the sarcoplasmic reticulum were locked open by treatment with ryanodine plus caffeine. After ryanodine treatment, the contractile responses to K(+) (3-20 mM) were augmented when compared with control responses in femoral and mesenteric arteries, but were inhibited in the carotid artery. Similar results were obtained when the contractile responses to norepinephrine were determined. The inhibition by ryanodine of the K(+)- or norepinephrine-contractions seen in the carotid artery was reversed by pretreatment with cyclopiazonic acid (10 microM), an inhibitor of the sarcoplasmic reticulum Ca(2+)-ATPase, but was not by charybdotoxin (100 nM), a blocker of Ca(2+)-activated K(+) channels. We conclude that (1) after ryanodine treatment, Ca(2+) entering from the extracellular space during stimulation with K(+) or norepinephrine is first taken up into the leaky sarcoplasmic reticulum and then reaches the myofilaments in femoral and mesenteric arteries, while in the carotid artery, Ca(2+) leaked from the sarcoplasmic reticulum reaches mainly the plasma membrane from where it is extruded into the extracellular space, and (2) the different movement of Ca(2+) may be due to the relative location of the sarcoplasmic reticulum in the smooth muscle cell of each artery.

Cyclopiazonic acid-induced changes in contractile activity of smooth muscle strips isolated from cat and guinea-pig stomach

The effects of cyclopiazonic acid (CPA), a specific inhibitor of sarcoplasmic reticulum Ca(2+)-ATPase, on contractile activity of circular smooth muscle strips isolated from the antrum, corpus and fundus regions of the cat and guinea-pig stomach were studied. Contractile activity was recorded under isometric conditions, in organ baths. CPA, concentration dependently (3 x 10(-7)-3 x 10(-5) M) increased the tone of the cat and guinea-pig gastric fundus and corpus as well as the amplitude of the phasic contractions of the cat corpus and antrum, affecting their frequency. CPA had a dual action on the phasic contractions of the guinea-pig antrum: an increase at low concentrations (up to 10(-6) M) and inhibition at high concentrations (10(-6)-3 x 10(-5) M). Tetrodotoxin (10(-6) M), atropine (10(-6) M) and N omega-nitro-L-arginine (10(-4) M) did not change significantly the effects of CPA. Nifedipine completely inhibited the CPA-induced phasic contractions and partly inhibited the CPA-induced tonic contractions. The nitric oxide-releasing agents, sodium nitroprusside (10(-3) M) and 3-morpholino-sydnonimine (10(-3) M), completely inhibited the CPA-induced tonic and phasic contractions. CPA induced tonic contractions in the cat and guinea-pig gastric fundus precontracted by acetylcholine (10(-5) M) and inhibited the acetylcholine (10(-6) M)-induced phasic contractions in the guinea-pig gastric antrum and corpus. The results suggest multiple roles for sarcoplasmic reticulum Ca2+ stores and sarcoplasmic reticulum Ca(2+)-ATPase in the shaping of spontaneous and evoked tonic and phasic contractions of the stomach, and highlight important species and tissue differences.

Possible mechanism of the potent vasoconstrictor actions of ryanodine on femoral arteries from spontaneously hypertensive rats

1. The Ca2+ buffering function of sarcoplasmic reticulum (SR) in the resting state of arteries from spontaneously hypertensive rats (SHR) was examined. Differences in the effects of ryanodine that removes the function of SR, on tension and cellular Ca2+ level were assessed in endothelium-denuded strips of femoral arteries from 13-week-old SHR and normotensive Wistar-Kyoto rats (WKY). 2. The addition of ryanodine to the resting strips caused a concentration-dependent contraction in SHR. This contraction was extremely small in WKY. In the presence of 10(-5) M ryanodine, caffeine (20 mM) failed to cause a further contraction in SHR, but it caused a small contraction in WKY. After washout of the strips with a Krebs solution, the resting tone was greatly elevated in SHR when compared with WKY. 3. The elevated resting tone in SHR strips was abolished by 10(-7) M nifedipine. The ryanodine-induced contraction was also abolished by 10(-7) M nifedipine. Nifedipine itself caused a relaxation from the resting tone of SHR strips, suggesting the maintenance of myogenic tone. 4. In strips preloaded with fura-PE3, the addition of 10(-5) M ryanodine caused a large and moderate elevation of cytosolic Ca2+ level ([Ca2+]i) in SHR and WKY, respectively. After washout, the resting [Ca2+]i was greatly elevated in SHR. The ryanodine-induced elevation of [Ca2+]i was decreased by 5 x 10(-6) M verapamil in SHR. Verapamil itself caused a decrease in resting [Ca2+]i which was significantly greater in SHR than in WKY, and caused a relaxation only in SHR. 5. The resting Ca2+ influx in arteries measured by a 5 min incubation with 45Ca was significantly increased in SHR when compared with WKY. The resting Ca2+ influx was not increased by 10(-5) M ryanodine in both SHR and WKY. The net cellular Ca2+ uptake in arteries measured by a 30 min incubation with 45Ca was decreased by 10(-5) M ryanodine in both strains. 6. The resting Ca2+ influx was decreased by 10(-7) M nifedipine in the SHR artery, but it was unchanged in the WKY artery. 7. These results suggest that (1) the Ca2+ influx via L-type voltage-dependent Ca2+ channels was increased in the resting state of the SHR femoral artery, (2) the greater part of the increased Ca2+ influx was buffered by Ca2+ uptake into the SR and some Ca2+ reached the myofilaments resulting in the maintenance of the myogenic tone, and (3) therefore the functional removal of SR by ryanodine caused a potent contraction in this artery.

Selective inhibition of myosin phosphorylation and tension of hyperplastic arteries by the kinase inhibitor HA1077

To examine possible alterations in myosin light chain phosphorylation in hyperplastic arteries, rabbit strips from right hyperplastic and left normal control carotid arteries were used for experiments 6 weeks after the ballooning procedure. When the hyperplastic artery was stimulated with various concentrations of K+ (10, 20, 30, 40 and 60 mM), the maximal tension in response to each concentration was significantly higher (P < 0.05) than that in the control artery. The maximal extent of myosin light chain phosphorylation induced by 60 mM K+ in the hyperplastic artery was also significantly higher than that in the control (55.1 +/- 4.1 vs. 45.1 +/- 3.2%, mean +/- S.D.). However, the [Ca2+]i response to elevated K+ in hyperplastic arteries was much the same as that in control arteries, when measured with fura-PE3. HA1077 (1-5-(isoquinolinesulfonyl)-homopiperazine), a protein kinase inhibitor, was about 3-5 times more effective in inhibiting the tension and myosin light chain phosphorylation induced by 60 mM K+ in the hyperplastic artery than in the control artery. Nifedipine inhibited the tension and myosin light chain phosphorylation to the same extent in control and hyperplastic arteries. Thus, an alteration of the myosin light chain phosphorylation system, but not an alteration of Ca2+ mobilization, may be involved in the enhanced contraction of the hyperplastic artery. The enhanced phosphorylation of myosin light chain may be sensitive to HA1077.

Superficial buffer barrier function of smooth muscle sarcoplasmic reticulum

In smooth muscle the superficial sarcoplasmic reticulum accumulates a portion of the Ca2+ that enters cells through the plasmalemma and thus functions as a buffer barrier to Ca2+ entry into the myoplasm (superficial buffer barrier or SBB). In this review Cornelis van Breemen, Qian Chen and Ismail Laher summarize experimental support for the SBB, and discuss data indicating that: (1) contraction is related more to the rate than extent of Ca2+ entry; (2) refilling of sarcoplasmic reticulum from the extracellular space is mediated by Ca2+ influx and Ca2+ pumping by the sarcoplasmic reticulum Ca2+ pump; (3) the superficial sarcoplasmic reticulum unloads Ca2+ to the extracellular space by a multi step process that involves sequentially the opening of Ca2+ and inositol 1,4,5-trisphosphate [Ins(1,4,5,)P3] sensitive channels and Ca2+ extrusion by Na(+)-Ca2+ exchange; (4) the SBB generates a peripheral Ca2+ gradient; (5) Ca(2+)-mobilizing receptor agonists generate Ins(1,4,5)P3 which short circuits the SBB to increase the effectiveness of Ca2+ influx in raising [Ca2+]i and consequently increase smooth muscle contraction. A physiologically regulated SBB is thought to enhance the informational content of Ca2+ signalling and support variable reduction of smooth muscle tone. Pharmacological modulation of Ca2+ transport in the superficial sarcoplasmic reticulum therefore presents an alternative means of controlling smooth muscle tone dependent on Ca2+ entry.