The functional significance of sodium channels in pancreatic beta‐cell membranes

Abstract

1. The existence and functional significance of Na channels in pancreatic β‐cell membranes were investigated by studying the effects of the plant alkaloid veratridine on the temporal release of insulin from perfused isolated rat islets of Langerhans. 2. 100 μ M veratridine evoked a sustained threefold increase in insulin release which was almost completely inhibited by 3 μ M tetrodotoxin (TTX). This action of TTX was rapidly reversible. 3. The simultaneous presence of 100 μ M propranolol, 100 μ M phenoxy‐benzamine and 10 μ M atropine did not alter the magnitude of the response to 100 μ M veratridine, indicating that the action of veratridine on the β‐cells was direct and was not mediated via the release of neurotrans‐mitters from nerve endings within the islets. 4. ⁴⁵Ca uptake by isolated islets in static incubation was increased almost threefold by 100 μ M veratridine. This increase was completely inhibited by the simultaneous presence of 3 μ M TTX. 5. Replacement of Na_o by choline caused a transient fourfold increase in insulin release which was associated with an increase in the uptake of ⁴⁵Ca from the extracellular space of similar magnitude. Subsequent exposure of islets to 100 μ M veratridine still evoked some insulin release but this only achieved 32% of that secreted by islets exposed to veratridine in medium of normal [Na]_o. 6. The addition of 2·5 m M CoCl₂ to the medium caused a 62·5% inhibition of veratridine‐mediated insulin release. 7. In Ca‐free medium supplemented with 1 m M EGTA, 100 μ M veratridine evoked insulin release of equal magnitude and of similar temporal relationship to that obtained in the presence of normal [Ca]_o. 8. A twofold increase in insulin release that occurred in the 15 min period immediately following exposure to 1 m M ouabain was completely independent of [Ca]_o. Subsequent ouabain‐evoked release became increasingly dependent on [Ca]_o. 9. Tetrodotoxin (3 μ M) inhibited the first phase of insulin release evoked by 16·7 m M d‐glucose by 37% and the second phase by 20%. 10. Both Na and Ca appear capable of entering through Na channels opened in the β‐cell membrane by veratridine. The increase in [Na]_i, resulting from the veratridine mediated increase in PNa⁺, causes depolarization of the β‐cell membrane with a consequent opening of voltage‐sensitive, Co²⁺‐blockable channels for additional Ca entry. An increase in [Na]_i also increases [Ca]_i by altering the equilibria of intracellular Ca‐sequestering mechanisms. The small but significant reduction of glucose‐mediated insulin release by TTX indicates that glucose has a rather weak action on the Na channel and a more pronounced effect on the voltage‐dependent Co²⁺‐blockable Ca channel.