Lanthanum and caffeine affect both membrane responses and force in a molluscan muscle, the radular protractor of Busycon canaliculatum

Abstract

1. Treatment with 5 mM lanthanum for a relatively brief time blocks both the rapid initial depolarization of the radular protractor by 10 −5 M ACh and the accompanying rapid contraction. Longer treatment blocks both sustained depolarization in ACh solution and the accompanying sustained force. 2. Lanthanum pretreatment for a time long enough to abolish the force accompanying depolarization by 100 mM KCl also reduces the rate of depolarization markedly. 3. Treatment with 10 mM caffeine immediately induces slow irregular oscillations in membrane potential difference, with superimposed fast membrane responses. Fluctuations in tonus, with superimposed fast contractions, faithfully follow the time course of electrical membrane activity. Relatively brief pretreatment with 5 mM lanthanum blocks both membrane and contractile effects of caffeine. It is thus not necessary to postulate that lanthanum penetrates and affects intracellular sites of action of caffeine. 4. The interaction of 10 −5 M acetylcholine (ACh) with a high concentration of 5-hydroxytryptamine (5HT) is complex but may result in potentiation of the ACh-response, with added depolarization and superimposed rhythmicity. 5. Lanthanum pretreatment blocks accompanying force, blocks the superimposed rhythmicity and blocks the added depolarization. After lanthanum treatment a repolarizing effect of 5HT predominates.