SUMMARY1. The f-action of catecholamines on lymphatic smooth muscle was studied by observing the effect of isoprenaline on electrical and mechanical activity in the double sucrose-gap.2. Action potentials and phasic contractions evoked by depolarizing pulses were abolished within 2 min of drug addition.3. Isoprenaline hyperpolarized the membrane and increased membrane conductance.4. Tetraethylammonium (10 mM) did not itself affect membrane resistance but reduced the hyperpolarization and the increase in conductance caused by isoprenaline.5. Removal of K+ from the external solution reduced membrane conductance and increased the hyperpolarization due to isoprenaline.6. When the NaCl content of Krebs solution was replaced with LiCl or choline chloride, isoprenaline no longer blocked action potential firing and its effects on phasic contractions and membrane conductance were reduced.7. In contrast, ouabain (10-5 M) did not block the effect of isoprenaline on membrane potential and membrane conductance.8. These results suggest that fl-adrenergic inhibition of lymphatic smooth muscle involves an increase in an outward K+ current, though an additional metabolic effect cannot be ruled out.
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.
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