Some properties of the smooth muscle of rabbit portal vein

Abstract

The morphology of the smooth muscle of the rabbit portal vein and its innervation were studied by fluorescence and electron microscopy. Two layers of smooth muscle were observed in the tunica media an inner layer of circularly arranged muscle cells and an outer layer consisting of bundles of smooth muscle cells arranged in a near longitudinal direction. The membranes of neighboring smooth muscle cells were occasionally fused to form ''tight junctions''. Bundles of non-myelinated nerve fibers were observed in the adventitia, and between bundles and layers of smooth muscle cells in the media. Studies on longitudinal sections with fluorescence microscopy revealed a network of varicose noradrenergic axons. Electrical and mechanical activity were recorded from longitudinal strips of smooth muscle from the media of the vein with a sucrose-gap apparatus. The preparation was spontaneously active under minimal resting tension (less than 150 mg) and at temperatures above 28[degree]C. Slow depolarizations led to a burst of spikes (multi-spike complexes), which corresponded to rhythmic contractions. In 19% of preparations, the interval between multi-spike complexes showed a slower depolarization, suggesting the record was from a pace-maker region. The frequency of spontaneous activity (3-27 beats/min.) was very sensitive to changes in temperature and tension. Noradrenaline [norepinephrine] in low doses (0.01 [mu]g) caused an increase in frequency of the multi-spike complexes. Higher doses (0.1-0.3 [mu]g) initiated continuous high-frequency spiking, while very high doses (0.6-2.0 [mu]g) caused maintained depolarization. Responses to repetitive electrical stimulation of the vein were qualitatively similar to those in response to exogenous norepinephrine. The relation between the mechanical response and the various parameters of stimulation was consistent with the stimulation of sympathetic nerve fibers in the wall of the vein. The actions of isoprenaline, phentolamine, and propranolol indicated the presence of a ''excitatory'' and [beta] ''inhibitory'' adrenotrophic receptors on the smooth muscle.