Excitation-contraction coupling in non-spiking smooth muscle in the gill ofAplysia

Abstract

1. Excitation-contraction coupling (E-C Coupling) was investigated in the non-spiking longitudinal efferent vein muscle (LEVM) from theAplysia gill. The sucrose-gap method was used to study the E-C coupling in response to acetylcholine (ACh) and to elevated external potassium (Ko). 2. LEVMs from mature and oldAplysia were used in this study. The response to high Ko and ACh was not significantly different in the two groups. 3. Depolarization increased directly with the concentration of either Ko (50 to 500 mM) or ACh (1 μM to 1 mM). It preceded the development of tension, which increased with depolarization. These stimulants did not evoke action potentials. 4. In the presence of MnCl2 (10–20 mM) for 30 min, ACh-contracture was reduced to near zero; in contrast K-contracture was only reduced to 60% of the control level; and depolarization was unaffected. In a nominally Ca-free medium for 30 min, ACh-contracture was reduced to zero, whereas K-concentration persisted at 16% of the control level. Hence, ACh and high Ko apparently mobilize different Ca stores to initiate E-C coupling: In the non-spiking LEVM, ACh-contracture primarily involves influx of Ca, whereas K-contracture appears to rely on primarily cellular Ca. The results are contrasted to those in a spiking molluscan muscle. 5. We propose that mobilizing both stores of calcium to initiate E-C coupling insures synchronized contraction of LEVM fibers. The similarity of activation in vitro, by Ko and ACh, and in situ, by neuronal firing, is shown and discussed.