Characterization of the ionic mechanism responsible for the hyperpolarization-activated current in frog sinus venosus

Voltage clamp experiments were carried out on the sinus venosus of the frog by means of the double mannitol gap technique. The ionic mechanism underlying the slowly hyperpolarization-activated inward current was investigated by changing the concentration and species of alkali cations and divalent cations in the bathing solution. Adding Rb or Cs in concentration of 10-20 mM to the control solution led to a dose-dependent increase in the inward current, as does elevating the external concentration of K from 2.5 to 25 mM. After the inward current had been nearly suppressed by completely substituting Tris for Na in the external medium, it was partially restored after a subsequent addition of K, Rb or Cs to the Na-free medium. Various alkaline earths or transition metals added to the bathing solution markedly depressed the magnitude of the inward current. This inhibitory effect varied with concentration and nature of divalent cations added. It also depended on the concentration and species of alkali cations present in the external solution. From these observations it was proposed that the conductance responsible for the inward rectification in frog sinus venosus does not discriminate among monovalent cations. The results support the existence of a weak-field-strength site located in the permeation pathway. Divalent cation may exert their inhibitory effect by competing with permeant ions for this site.