Ionic interactions in the smooth muscles of cat pulmonary artery and portal vein

The contractile activity of isolated smooth-muscle preparations of pulmonary artery and portal vein was recorded under isometric conditions and the changes in the membrane potential were recorded by the sucrose gap method. The replacement of Na+ by sucrose or cholinechloride ([Na+]o = 0) in the presence of phentolamine and atropine causes contraction of the smooth-muscle strips, characterized by initial fast and subsequent slow tonic component. The contraction is accompanied by hyperpolarization of the membrane. D600 (10(-5) M) and sodium nitroprusside (10(-5) M) do not influence the tonic component of the [Na+]o = 0-induced contraction. Low Na-content in the solution ([Na+]o = 15.5 mM) causes brief contraction only of the portal vein strips. In Ca-free solution the tone of the arterial preparations does not change, while the tone of portal vein strips decreases and phasic contractions are inhibited. In Ca-free solution or after pretreatment of the strips with 10 mM LaCl3, substitution of sodium induces a slowly developing contraction without initial fast component. Pretreatment of the preparations with ouabain (10(-4) M) or their preincubation in K-free solution potentiates the [Na+]o = 0-induced contraction. The results obtained exclude membrane-potential-dependent processes in the development of the [Na+] o = 0-induced contraction and support the existence of Na+-Ca2+ exchange mechanism in the smooth muscles of cat pulmonary artery and portal vein.