Membrane properties of smooth muscle cells in pulmonary hypertensive rats

The membrane properties of smooth muscle cells in rat main pulmonary artery (MPA) and small pulmonary artery (SPA) were investigated during chronic normobaric hypoxia and after monocrotaline injection. As chronic pulmonary hypertension developed, pronounced differences between MPA and SPA were observed. These findings may shed light on mechanisms of smooth muscle hypertrophy. 1) The resting membrane potential of smooth muscle in MPA became less negative than the normal (depolarized), whereas the resting membrane potential of smooth muscle in SPA became more negative (hyperpolarized). 2) In MPA, both the length and time constants diminished. 3) In MPA, the maximum membrane depolarization produced by a 10-fold increase in extracellular [K+] decreased. 4) In SPA, the depolarization observed in K+-free solution was more rapid and greater in amplitude, and the transient hyperpolarization following restoration of K+-containing solution increased. 5) In SPA, initial and sustained depolarization evoked by Na+-deficient solutions were increased. 6) Depolarization in MPA was due to increased membrane permeability, perhaps to Cl-, whereas hyperpolarization in SPA could be attributed to increased activity of an electrogenic Na+-K+ pump.