Importance of calcium for the stimulant action of norepinephrine and acetylcholine on the guinea pig myometrium

The results to be described were obtained from the myometrium of immature guinea pigs which had been treated by daily injections of 5 μg estradiol for 8 days. The double sucrose gap was used to record changes in membrane potential, membrane resistance and isometric tension. In Krebs solution after the initial α stimulatory effect of norepinephrine (NE), there was an increase in the force of the phasic contraction which was unaccompanied by depolarization or increase in spike frequency. This positive inotropic response to NE was observed in K free solution, Na free solution, and Cl deficient solution. Since Ca²⁺ carries the current for the evoked spike, these results suggest that NE increases the inward Ca²⁺ current carried by the voltage dependent channels. In K depolarizing solution, Cl deficient solution and in modified Krebs solution containing lanthanum, NE increased tension suggesting that NE may release Ca²⁺ from intracellular stores. Ca²⁺ also participates in the stimulant action of acetylcholine (ACh). In Na free solution, ACh caused a depolarization, an increase in conductance, and a contracture. When Ca²⁺ was removed, ACh produced a hyperpolarization, a smaller increase in conductance and a contracture. These data suggest that Ca²⁺ can substitute for sodium ions in the conductance increase caused by ACh and that ACh can release Ca²⁺ from intracellular stores.