Action of oxytocin on mammary gland: influence of hypotonicity

The enhancement of the response of an isolated strip of lactating mouse mammary gland to oxytocin, resulting from dilution of the Tyrode solution in which oxytocin is administered, is due to the effects of hypotonicity per se and not to changes in concentration of any of the ions in the bathing solution. Inhibition of membrane ATPase also causes an enhancement, which is probably due to activation of a secondary facilitating mechanism, rather than to enhancement of the entire train of oxytocin-induced reactions. A variety of conditions which alter the response to oxytocin have the same effect on the response to acetylcholine. Hypotonicity can therefore increase the sensitivity of this bioassay for oxytocin, but not its selectivity.