Intracellular Ca2+ and contractile responses to alpha 1-adrenoceptor subtype activation in rat aortic vascular smooth muscle

Biochemical and pharmacological characterization of toxins obtained from the fire coral Millepora complanata

Millepora complanata is a normal resident of coral reefs in the Mexican Caribbean. In this study, we describe for the first time the vasoconstrictor, phospholipase A2 (PLA2), and hemolytic activities elicited by a crude extract obtained from M. complanata. This extract caused a concentration-dependent contraction of isolated rat aortic rings (EC50=22.4+/-1.1 microg protein/mL). This effect was endothelium independent and significantly reduced in the absence of extracellular Ca2+ and when the intracellular Ca2+ stores were depleted. In addition, the crude extract obtained from M. complanata showed PLA2 activity (7.231+/-0.092 mmol min(-1) mg(-1)) and hemolysis of rat erythrocytes (HU50=1.64+/-1.04 mug protein/mL). The hemolysis increased in the presence of Ca2+ and decreased in the presence of cholesterol. Furthermore, this hemolysis was significantly reduced after incubation with an inhibitor of PLA2 enzymes. The hemolytic and vasoconstrictor effects were abolished after incubating the extract under denaturing conditions. Reverse phase chromatography of the M. complanata extract afforded 19 fractions (F1 to F19). F4 induced hemolysis and contained mainly a protein of 30 kDa, probably a PLA2 enzyme, while F8 and F11, containing mainly proteins of 15 and 20 kDa respectively, produced vasoconstrictor effects mediated by different mechanisms of action.

Mechanism of acidic pH-induced contraction in spontaneously hypertensive rat aorta: role of Ca2+release from the sarcoplasmic reticulum

AIM

This study was conducted to investigate the mechanism of acidic pH-induced contraction (APIC) with regard to Ca2+ handling using isometric tension recording experiments.

RESULTS

Decreasing extracellular pH from 7.4 to 6.5 produced a marked and sustained contraction of spontaneously hypertensive rat (SHR) aorta, that was 128.7 +/- 2.0% of the 64.8 mm KCl-induced contraction. Verapamil, an inhibitor of voltage-dependent Ca2+ channels (VDCC) significantly inhibited the APIC. In Ca2+-deficient solution, sustained contraction induced by acidic pH was abolished completely, while a transient contraction was still observed suggesting the release of Ca2+ from intracellular site. Ryanodine (1 microm), a ryanodine receptor blocker, and 10 microm cyclopiazonic acid (CPA; a sarco/endoplasmic reticulum Ca2+ ATPase inhibitor) abolished the transient contraction induced by acidosis. In normal Ca2+-containing solution, ryanodine significantly decreased the rate of rise as well as maximum level of APIC. Interestingly, ryanodine and CPA showed an additive inhibitory effect with verapamil and the combined treatment of ryanodine or CPA with verapamil nearly abolished the APIC.

CONCLUSIONS

It is concluded that acidic pH induces Ca2+ release from ryanodine/CPA-sensitive store of sarcoplasmic reticulum in SHR aorta. This Ca2+ plays an important role in the facilitation of the rate of rise of APIC, as well as contributing to the sustained contraction via a mechanism which is independent of Ca2+ influx through VDCC.

Hypoxia inhibits contraction but not calcium channel currents or changes in intracellular calcium in arteriolar muscle cells

OBJECTIVE

We tested the hypothesis that hypoxia inhibits currents through L-type Ca(2+) channels and inhibits norepinephrine-induced rises in intracellular Ca(2+) in cremasteric arteriolar muscle cells, thus accounting for the inhibitory effect of hypoxia on norepinephrine-induced contraction of these cells.

METHODS

Single smooth muscle cells were enzymatically isolated from second-order and third-order arterioles from hamster cremaster muscles. The effects of hypoxia (partial pressure of oxygen: 10-15 mm Hg) were examined on Ba(2+) (10 mM) currents through L-type Ca(2+) channels by use of the perforated patch clamp technique. Also, the effect of hypoxia on norepinephrine-induced calcium changes was studied using Fura 2 microfluorimetry.

RESULTS

Hypoxia inhibited the norepinephrine-induced (10 microM) contraction of single arteriolar muscle cells by 32.9 +/- 5.6% (mean +/- SE, n = 4). However, hypoxia had no significant effect on whole-cell currents through L-type Ca(2+) channels: the peak current densities measured at +20 mV were -3.83 +/- 0.40 pA/pF before hypoxia and -3.97 +/- 0.36 pA/pF during hypoxia (n = 15; p > 0.05). In addition, hypoxia did not inhibit Ca(2+) transients in arteriolar muscle cells elicited by 10 microM norepinephrine. Instead, hypoxia increased basal Ca(2+) (13.8 +/- 3.2%) and augmented peak Ca(2+) levels (29.4 +/- 7.3%) and steady-state Ca(2+) levels (15.2 +/- 5.4%) elicited by 10 microM norepinephrine (n = 21; p < 0.05).

CONCLUSIONS

These data indicate that hypoxia inhibits norepinephrine-induced contraction of single cremasteric arteriolar muscle cells by a mechanism that involves neither L-type Ca(2+) channels nor norepinephrine-induced Ca(2+) mobilization. Instead, our findings suggest that hypoxia must inhibit norepinephrine-induced contraction by affecting a component of the signaling pathway that lies downstream from the increases in Ca(2+) produced by this neurotransmitter.

Mechanism of internal anal sphincter smooth muscle relaxation by phorbol 12,13-dibutyrate

We investigated the mechanism of the inhibitory action of phorbol 12,13-dibutyrate (PDBu), one of the typical protein kinase C (PKC) activators, in in vitro smooth muscle strips and in isolated smooth muscle cells of the opossum internal anal sphincter (IAS). The inhibitory action of PDBu on IAS smooth muscle (observed in the presence of guanethidine + atropine) was partly attenuated by tetrodotoxin, suggesting that a part of the inhibitory action of PDBu is via the nonadrenergic, noncholinergic neurons. A major part of the action of PDBu in IAS smooth muscle was, however, via its direct action at the smooth muscle cells, accompanied by a decrease in free intracellular Ca(2+) concentration ([Ca(2+)](i)) and inhibition of PKC translocation. PDBu-induced IAS smooth muscle relaxation was unaffected by agents that block Ca(2+) mobilization and Na+-K+-ATPase. The PDBu-induced fall in basal IAS smooth muscle tone and [Ca(2+)](i) resembled that induced by the Ca(2+) channel blocker nifedipine and were reversed specifically by the Ca(2+) channel activator BAY K 8644. We speculate that a major component of the relaxant action of PDBu in IAS smooth muscle is caused by the inhibition of Ca(2+) influx and of PKC translocation to the membrane. The specific role of PKC downregulation and other factors in the phorbol ester-mediated fall in basal IAS smooth muscle tone remain to be determined.