In vitro studies of frog mucous glands

The ionic outflow, mainly consisting of Na+ and Cl-, from the mucous glands in an excised nerve-skin preparation of frog has been determined by recording the conductance changes occurring in a fluid layer covering a small area of the skin surface. In the main series of experiments the glands were activated by stimulation of sympathetic nerve fibers in the skin nerve. The relationship between the ionic outflow and the number of nerve volleys was studied over a wide range. The outflow per impulse was found to be fairly constant during the first tens of impulses but diminished gradually with increasing number of stimuli up to a certain maximum value--varying in different preparations--after which the outflow ceased completely. During the initial phase of stimulation the outflow is most likely caused by an ejection of performed secretion due to the contractions of the glandular myoepithelium. The continued outflow in the later stages of the stimulation periods must be due to production of new secretion. Since the glandular epithelium is devoid of nerve terminals a nervous control of the ionic secretion can only be explained by an indirect influence mediated either by transmitter diffusion from the myoepithelial nerve endings or by a close electric coupling between the contractile and the secretory gland cells. Adrenaline and noradrenaline induce ionic outflows which like those evoked by nerve stimulation are inhibited by the beta-adrenoreceptor blocker propranolol, alpha-adrenoreceptor blockers being without effect. A serendipitous finding of tonus changes in the frog skin during nerve stimulation is also described.