Excitation-contraction coupling and intracellular calcium pool in the penis retractor muscle of Helix pomatia

Abstract

1. The effect of Adalat, D-600, Verapamil and LaCl 3 reported to be antagonists of inward Ca 2+ current were tested on the penis retractor muscle (PRM) of Helix pomatia. 2. The spontaneous electrical and mechanical activity observed in this muscle preparation were diminished or abolished in the presence of “Ca 2+-antagonists”. D-600 was found to be the most potent inhibitory agent. 3. The ACh-induced contraction and the contracture in response to KCl containing depolarizing solution were blocked by “Ca 2+-antagonists”, while the sustained stage of the ACh-contraction was unaffected. 4. Exposure to “Ca 2+-antagonists” prior to caffeine stimulation abolished the second slow component of the caffeine-contracture, while the initial fast tension development could still be elicited. 5. The results suggest that the KCl-contracture as well as the tension development in response to ACh involve external Ca 2+ for direct or indirect activation of the contractile apparatus, while the sustained stage of the ACh-contraction and the initial fast component of the caffeine-contracture are independent of external Ca 2+ indicating that intracellular Ca 2+ pools are present in the PRM. 6. Electronmicroscopy shows four possible candidates for intracellular Ca 2+ storage sites.