Characteristics of hyperpolarization-activated cation currents in portal vein smooth muscle cells

Abstract

Voltage-clamp studies of freshly isolated smooth muscle cells from rabbit portal vein revealed the existence of a time-dependent cation current evoked by membrane hyperpolarization (termed I h). Both the rate of activation and the amplitude of I h were enhanced by membrane hyperpolarization. Half-maximal activation of I h was about −105 mV with conventional whole cell and −80 mV when the perforated patch technique was used. In current clamp, injection of hyperpolarizing current produced a marked depolarizing “sag” followed by rebound depolarization. Activation of I h was augmented by an increase in the extracellular K+ concentration and was blocked rapidly by externally applied Cs+ (1–5 mM). The bradycardic agent ZD-7288 (10 μM), a selective inhibitor of I h, produced a characteristically slow inhibition of the portal vein I h. The depolarizing sag recorded in current clamp was also abolished by application of 5 mM Cs+. Cs+ significantly decreased the frequency of spontaneous contractions in both whole rat portal vein and rabbit portal vein segments. Multiplex RT-PCR of rabbit portal vein myocytes using primers derived from existing genes for hyperpolarization-activated cation channels (HCN1–4) revealed the existence of cDNA clones corresponding to HCN2, 3, and 4. The present study shows that portal vein myocytes contain genes shown to encode for hyperpolarization-activated channels and exhibit an endogenous current with characteristics similar to I h in other cell types. This conductance appears to determine, in part, the rhythmicity of this vessel.