Mechanism of Alpha-adrenergic Excitation in Bovine Lymphatic Smooth Muscle

Measurements were made, using the double sucrose-gap technique, of electrical and mechanical responses of bovine lymphatic smooth muscle to constant current pulses. After beta-blockade with 10(-6) M propranolol, stimulation of the alpha-receptors with norepinephrine (5 X 10(-6) M) depolarized the membrane and decreased membrane conductance. The depolarization and decrease in membrane conductance persisted in Li+- and choline-substituted low-Na+ solution, in methanesulfonate-substituted low-Cl- solution, and in Ca2+- free solution containing 1 mM ethyleneglycol-bis(beta-aminoethylether)-N, N'-tetraacetic acid. Tetraethylammonium (10 mM) did not itself affect membrane resistance nor did it block the increase in resistance due to norepinephrine. In contrast, cesium (10 mM) increased membrane resistance and prevented norepinephrine from increasing this further. As well as these effects on membrane resistance, norepinephrine (5 X 10(-6) M) increased the duration of the action potential, and this was accompanied by an increased force of contraction. Tetraethylammonium prolonged the action-potential plateau and potentiated norepinephrine's effect. These results suggest that norepinephrine is likely to increase the efficiency of lymphatic pumping due to both its positive inotropic effect and the improved safety margin for propagation resulting from the increase in membrane resistance. The latter effect may be due to the suppression of an outward K+ current.