Inward rectification in freshly isolated single smooth muscle cells of the rabbit jejunum

1. Single smooth muscle cells were obtained by collagenase digestion of longitudinal muscle of rabbit jejunum. Membrane potential or current under voltage clamp was recorded by patch-clamp technique in the whole-cell recording mode at 22-25 degrees C. 2. At a membrane potential of -50 mV small hyperpolarizing current pulses produced electrotonic potentials which asymptotically approached a steady-state size and did not 'sag'. Stronger hyperpolarization resulted in a 'sag' of the electrotonic potential. Under voltage clamp an inward current, i, was activated in the range -60 to -110 mV. 3. This current had an equilibrium potential of -24.5 +/- 3.5 mV which was shifted negatively by reducing the sodium concentration, and positively by raising the potassium concentration of the bathing solution. 4. This current was blocked by caesium (1 mM) but less affected by barium ions up to 10 mM. 5. The time course of i upon stepping into its activation range was accelerated by increasing negativity and following an initial short delay could be described by a single exponential with a time constant in the range 60 s(-60 mV) to 1 s(-130 mV). 6. It is concluded that these jejunal cells possess a current to which sodium and potassium ions make a contribution which is responsible for the inward rectification they show upon hyperpolarization. This current is activated in a range which would allow it to contribute to the slow potential changes shown by longitudinal jejunal muscle.