Dissociation of contraction and potassium efflux in smooth muscle

Normally, a pilocarpine induced contraction of the isolated guinea pig ileum is associated with a change in potassium transport across cell membranes. Bathing the tissue in a Ca-free medium will effect a dissociation between the contractile response and the potassium efflux. The replacement of Tyrode's solution by a Ca-free medium elicits a slight increase in smooth muscle tone followed by a return to a state of relaxation. Within an hour the contractile response to pilocarpine becomes negligible. This lack of response to pilocarpine applies to circular as well as longitudinal smooth muscle. In contrast to the changes which occur in muscle tone the efflux of potassium, following withdrawal of Ca ion, undergoes a sudden rise and remains above normal. The administration of pilocarpine, after the tissue has been bathed in the Ca-free medium for 1 or 2 hours, will still evoke a moderate additional increase in potassium efflux. Replacement of Ca ion with an equimolar quantity of Mg ion causes the contractile response to pilocarpine to disappear sooner than it would have in a simple calcium-free medium. Here, too, the potassium efflux of the tissue rises and after 1 hour responds to pilocarpine with a further increase. Thus, pilocarpine can modify potassium transport in the isolated ileum regardless of its effect on smooth muscle tone.