Characterization of properties underlying rhythmicity in mouse portal vein.

We have used sharp intracellular and patch clamp electrophysiology, together with mechanical recordings and immunohistochemistry to characterize some of the properties underlying spontaneous rhythmicity in isolated murine portal vein. Mechanical recordings revealed that isolated whole portal veins were spontaneously active and generated regular contractions every 5-15-s that persisted in the presence of cyclopiazonic acid (CPA) (10 microM) or thapsigargin (100 nM). Intracellular recordings from smooth muscle cells revealed spontaneous depolarizations (SDs) in membrane potential, which were abolished by nifedipine (1 microM). Whole cell patch clamp recordings from isolated smooth muscle cells revealed an inward "pacemaker" current (I(H)) at negative potentials. Immunohistochemical studies failed to detect the presence of Kit-immunoreactive cells in portal veins of wild type mice, but were consistently observed in the small intestine. Furthermore, portal veins obtained from W/W(v) mutant mice, which lack full expression of the tyrosine-kinase, c-Kit, were also rhythmically active and were not different from wild type mice, in either their electrical or mechanical properties. These results show that both the wild type and W/W(v) mutant mouse portal vein are rhythmically active in vitro. However, pacemaker activity in this blood vessel occurs in the absence of Kit-immunoreactive cells; and is not critically dependent upon release of Ca(2+) from intracellular stores. The rhythmic pacemaker activity of mouse portal vein does involve L-type Ca(2+) currents, and possibly pacemaker conductances intrinsic to the smooth muscle.