Carbachol-induced contraction in the circular muscle of guinea-pig stomach in calcium-free solution

Origin of Ca2+ necessary for carbachol-induced contraction in longitudinal muscle of the proximal colon of rats

The origin of Ca2+ necessary for carbachol (CCh)-induced contraction of longitudinal muscle of the proximal colon of rats was studied. CCh induced contraction of the muscle consisting of two phases, phasic and tonic phases, with a concomitant biphasic increase in [Ca2+]i. After removal of Ca2+ from the bathing solution of the colonic segments, CCh-induced contraction was rapidly inhibited; there was almost complete inhibition 1 min after the removal. Nicardipine, a blocker of voltage-dependent calcium channel, also significantly inhibited CCh-induced contraction. On the other hand, treatment of the colonic segments with thapsigargin, an inhibitor of sarcoplasmic reticulum (SR) Ca2+-ATPase, did not significantly affect the contraction except causing a slight decrease in the rate of contraction. These results suggest that Ca> entering through voltage-dependent calcium channels, but not released from SR, is essential for CCh-induced contraction of longitudinal muscle of the proximal colon of rats. This strict dependency of the CCh-induced contraction on extracellular Ca2+ was discussed in relation to the results obtained in the fundus of rats.

Membrane-associated signal transduction modulates contractile responses to Ca2+ in saponin-skinned coronary smooth muscle

Experiments on saponin-permeabilized bovine and porcine coronary arteries were performed to examine whether fluoride-induced contractile mechanisms are retained after permeabilization and whether they modulate muscle tension without a change in cytoplasmic Ca2+. Fluoride induced reversible contractions in skinned coronary smooth muscle and shifted the apparent Ca2(+)-sensitivity of the contractile system to lower Ca2+ values. Force development to fluoride required exogenous ATP and a minimum Ca2+ concentration (pCa less than 8) of the bathing medium. Force development occurred when the concentration of free Ca2+ available to the contractile apparatus seemed to be sufficiently controlled by a Ca2(+)-EGTA buffering system. The effects of fluoride were enhanced by Al3+ and imitated by both guanylyl-imidodiphosphate and 4 beta-phorbol-12,13-dibutyrate which suggests the involvement of both GTP-binding protein(s) and of the C-kinase pathway. Under conditions of sufficiently controlled Ca2+ buffering, saponin-skinned arteries responded to histamine or carbachol with an increase in muscle tension which was inhibited by specific receptor blocking agents (diphenhydramine, atropine), and which required the presence of both exogenous ATP and a low concentration of Ca2+ (pCa 7) which per se did not result in a contractile response. It is concluded that agents acting on different steps of membrane-associated signal transduction pathways are able to induce contractile responses of saponin-skinned vascular smooth muscle without a change in cytosolic Ca2+ concentration.

Calcium reversal, relaxation by calcium ion of various smooth muscles contracted by carbachol, norepinephrine or a phorbol ester in calcium-ion free medium

1. Ca reversal was reported on the longitudinal smooth muscle of rat uterus and the fundic part of the circular smooth muscle of guinea pig stomach; that is, addition of Ca ion to the Ca-free bathing solution in which the muscle is contracting to oxytocin or carbachol, respectively, relaxes the muscle. 2. We tested whether this inhibitory action of Ca ion is seen in other smooth muscles including vascular, air way, gastric and genital smooth muscles. 3. We found that Ca reversal was observed in the smooth muscle of rat thoracic aorta contracted by norepinephrine or by a phorbol ester, TPA, guinea pig tracheal smooth muscle contracted to carbachol, fundic smooth muscle of rat stomach to carbachol, corporal and antral smooth muscle of guinea pig stomach to carbachol and rat vas deferens to norepinephrine. 4. Ca reversal was observed not only when the muscle contracted by an agonist of the surface receptor such as oxytocin, norepinephrine or carbachol but also by a phorbol ester that is believed to cause contraction in the cell by activating phosphorylation. 5. Thus, we conclude that Ca reversal is a universal phenomenon in smooth muscles.

Effects of ouabain and EGTA on the inhibitory effect of Ca2+ on guinea pig stomach smooth muscle ('Ca reversal')

Guinea pig stomach smooth muscle is known to show sustained contraction when treated with carbachol (CCh) in Ca-free solution containing 2 X 10(-3) M EGTA ('Ca-free contraction'). After pretreatment of the muscle with a Ca antagonist (D-600, nicardipine or nifedipine), addition of 10(-4) M Ca2+ (pCa 4.0) relaxed this CCh-induced Ca-free contraction ('Ca reversal'), but without pretreatment with a Ca antagonist, Ca2+ evoked contraction. In a Ca buffer containing 2 X 10(-3) M EGTA, low concentrations of Ca2+, 10(-8)-3 X 10(-7) M (pCa 8.0-6.5), relaxed the Ca-free contraction dose dependently ('Ca reversal'), whereas concentrations higher than 10(-6) M Ca2+ (pCa 6.0-) reversed this relaxation back to contraction. EGTA at 5 X 10(-3) or 10(-2) M potentiated the Ca reversal, whereas no reversal occurred in a Ca buffer containing 10(-3) M EGTA. This Ca reversal was inhibited in K(+)-depleted, Ca-free solution and by ouabain in Ca buffer containing EGTA (5 X 10(-3) or 10(-2) M). These results show that Ca reversal of guinea pig stomach smooth muscle occurs at low concentrations of Ca2+ and that its mechanism is related to ouabain-sensitive processes, such as the function of Na(+)-K+ ATPase.

"Ca reversal": inhibition by submicromolar Ca2+ concentrations of contraction of smooth muscle and its use for assay of Ca channel blockers

1. Ca ion, which is a trigger of many physiological responses, also acts as an inhibitor of responses at lower concentrations than those that act as triggers in rat uterine smooth muscle. 2. The contraction induced by oxytocin of uterine smooth muscle that had been preincubated in Ca-free solution with 3 mM EGTA for 1 hr was reversed by addition of a resting level of Ca ion (10(-8) - 10(-7) M). 3. This relaxation by Ca ion was named "Ca reversal". 4. This finding suggests that Ca ion at low concentrations has an inhibitory action. 5. Since Ca reversal is manifested in the presence of Ca antagonists, we tested the availability of this phenomenon for assay of Ca antagonistic activity. 6. This system is very simple because the judgement is all or none, and it is very useful because other antispasmodic actions can be determined simultaneously. 7. The ED50 values of various drugs for Ca reversal show their Ca antagonistic activities. 8. Furthermore, the value in high K+ solution shows the selectivity of a Ca antagonist for voltage-operated Ca channels. 9. Thus this system is very useful for assay of Ca antagonistic activities, which are very important for drug therapy in a wide variety of diseases.

Effects of low extracellular Ca2+ on prostaglandins F2 alpha and E2 action in longitudinal muscle of guinea-pig caecum

1. Effects of low extracellular Ca2+ on the action of prostaglandin F2 alpha (PGF2 alpha) and prostaglandin E2 (PGE2) in longitudinal muscle of guinea-pig caecum were evaluated by the changes in the membrane potential, action potential frequency and contractions. 2. At 2.5 mmol extracellular Ca2+ the EC50 values for the stimulant effects of PGF2 alpha and PGE2 on the membrane potential, the frequency of action potential discharge and contractions were higher for PGF2 alpha than the PGE2. 3. The lowering of extracellular Ca2+ from 2.5 mmol to 0.4 mmol did not affect essentially the PGF2 alpha and PGE2 effects. 4. In Ca2+-free solution containing EGTA the stimulant effects of PGF2 alpha and PGE2 significantly decreased but the action potentials and contractions were not completely abolished. D-600 pretreatment inhibited in a dose-dependent manner the PGF2 alpha- and PGE2-evoked action potential discharges and contractions. 5. It is concluded that the mechanisms limiting Ca2+ influx at 2.5 mmol extracellular Ca2+ are more sensitive to PGE2 than to PGF2 alpha. 6. At low extracellular Ca2+ and in Ca2+-free solution containing EGTA the effects of PGF2 alpha and PGE2 on the action potential generation and the corresponding contractions might be determined by Ca2+-independent processes.