Contraction, membrane potential, and calcium fluxes in rabbit pulmonary arterial muscle

The rabbit main pulmonary artery (RMPA) has frequently been used for studies of contraction, membrane properties, and ion fluxes. The resting membrane potential (Em) of the smooth muscle cells of the RMPA is close to -60 mV. The diffusion potential calculated from ion concentrations and permeabilities is -31 to -40 mV, which suggests that electrogenic ion pumping contributes to the actual Em. Circumferential strips of RMPA possess cablelike properties with a space constant lambda of 1.9 mm. Contraction of RMPA to high K+ depends on extracellular Ca2+, is associated with 45Ca influx, is inhibited by Ca2+ entry blockers, and occurs after depolarization of the membrane to -45 to -33 mV. Maximal contractile responses to K+ and norepinephrine (NE) were similar. At low concentrations (3 X 10(-8)-10(-6) M) NE and the alpha 1-agonist methoxamine induced concentration-dependent depolarization and contraction. Above 10(-6) M contraction occurred in the absence of further changes in Em. Membrane resistance, estimated from measurements of space constant, decreased over the entire concentration-contraction curve of alpha agonists. Blockade of potassium channels by tetraethylammonium unmasked depolarization at high NE concentrations. It is concluded that in the RMPA alpha 1-adrenoceptor stimulation is associated with changes in electrical membrane properties and may in this way trigger contraction.