Enhancement of ornithine decarboxylase activity in Ambystoma liver slices by ovine prolactin: An evaluation of possible mediators

Abstract

Prolactin has been shown to increase the activity of ornithine decarboxylase in a variety of mammalian tissues and in the pigeon crop sac. This study demonstrates a similar effect of ovine prolactin on ornithine decarboxylase activity in liver slices taken from larval tiger salamanders ( Ambystoma tigrinum). An evaluation of potential mediators of prolactin action in liver slices revealed that the effect of the hormone on enzyme activity was not blocked by ouabain, an inhibitor of the sodium pump reported to block other actions of prolactin. Oxytocin, which inhibits prolactin actions in A. tigrinum, blocked the increase in ornithine decarboxylase activity induced by prolactin. Since previous results had implicated inositol phospholipid turnover in oxytocin action, the effects of the calcium ionophore, A 23187, and of synthetic diacylglycerol were examined. Both agents blocked the increase in enzyme activity when they were combined with prolactin treatment. Verapamil, a calcium channel blocker, had a prolactin-like effect on the activity of ornithine decarboxylase, and the combination of prolactin and verapamil produced a stimulation of the enzyme that was no greater than that observed with either the drug or prolactin alone, suggesting that both agents might be acting via a common cellular pathway. The tentative hypothesis that prolactin acts via a mechanism which lowers intracellular calcium is suggested.