Electrical and mechanical activities in the radula protractor of a mollusc,Rapana thomasiana

Electromechanical uncoupling in a molluscan muscle examined by the sucrose gap technique. The effect of calcium antagonist and agonist agents

Membrane potential and tension of Busycon radular protractor muscles were studied by sucrose gap methods. Excitation-contraction (EC) coupling was examined in response to acetylcholine (ACh) and high K which depolarized the fibres and induced tension, but without action potential firing. Potassium depolarization did not follow predictions expected from the Nernst equation at low and very high K levels, and maximum tension was found at about 100 mM K. EC coupling was very sensitive to [Ca]0. Ca-free media eliminated K- and ACh-induced tension but with normal depolarization, showing full electromechanical uncoupling. Ionophore A23187 enhanced K- and ACh-induced responses and X-537A enhanced ACh responses, demonstrating acute dependence of activation on [Ca]0 in this muscle. The calcium antagonists nifedipine and nisoldipine reduced tension in the muscle only at very high concentrations, and both agents slightly reduced K- and ACh-induced depolarization. Verapamil reduced K- and ACh-induced tension but paradoxically it enhanced the depolarizing actions of these agents leading to electromechanical uncoupling. Abscisic acid (ABA) enhanced ACh- and K-induced tension and simultaneously enhanced their depolarizing actions. Ionophores and ABA appear to enhance calcium influx which may secondarily influence sodium influx. Calcium antagonists have no consistent actions on this muscle, suggesting that calcium channel activity of the radular protractor may be different from that seen in mammalian visceral muscles.

Antagonistic responses of the radular protractor and retractor to the same putative neurotransmitters

1. Mechanisms of antagonistic responses of the radular protractor and the retractor to the same neurotransmitters, octopamine and serotonin, were investigated in a mollusc, Rapana thomasiana. 2. Hyperpolarization induced by octopamine or serotonin was found to depend mainly on the increase in Cl- conductance of the membrane in both the radular protractor and the retractor. 3. In the protractor, the enhancement of acetylcholine depolarization during the application of octopamine or serotonin was exhibited even when the membrane potential was varied by current application. Spontaneous firing was blocked by these amines, but larger spikes with higher frequency were elicited on giving a depolarizing current above threshold. 4. In the retractor, the inhibition of glutamate depolarization during the application of octopamine or serotonin was observed irrespective of membrane potential. Spontaneous spikes were stopped by these amines but reappeared on depolarization above threshold. Reappeared spikes during serotonin application were smaller than control spikes, while those during octopamine application were about the same as or larger than control ones. 5. The modes of actions of octopamine and serotonin on both muscles were considered.

Physiological properties of the penis retractor muscle of Aplysia

The properties of the penis retractor muscle of Aplysia have been studied using intracellular, sucrose gap and tension recording. The fibers are of the invertebrate smooth muscle type and exhibit slow contractions which occur spontaneously or in response to stretch in isolated preparations. Individual muscle fibers are innervated by excitatory and inhibitory axons. A variety of sizes of excitatory and inhibitory junctional potentials can be recorded from them. The innervation is probably diffuse and functionally polyneuronal. The fibers are electrically coupled, permeable to potassium and chloride at rest, and exhibit no overshooting active responses. The muscle shows graded responses of depolarization and contraction proportional to strength of nerve stimulation. Facilitation and depression of junctional potentials are seen with various frequencies of nerve stimulation. Post-tetanic potentiation occurs with nerve stimulation at frequencies from 2 to 50 Hz and is suppressed in the presence of increased extracellular calcium concentrations.