Hyperpolarization-activated cationic channels in smooth muscle cells are stretch sensitive

The properties of hyperpolarization-activated channels were studied in single smooth muscle cells from the stomach of the toad, Bufo marinus, using the patch-clamp technique. In cell-attached patches, inward channel currents were activated by hyperpolarizing pulses from a holding potential of -20 mV to potentials more negative than -60 mV. The activity of the channels increased and their latency of activation decreased as the hyperpolarization was increased. The slope conductance of the channels with standard high sodium concentration pipette solution was 64.2 +/- 9.1 pS (SD, n = 17). Stretching the patch, by suction applied to the back of the patch pipette, also increased the activity and shortened the latency of activation. We designate these channels as HA-SACs (hyperpolarization- and stretch-activated channels). HA-SACs were observed in 83% (175/210) of the patches studied. HA-SAC currents were carried by sodium and potassium ions, but their amplitude was increased by replacing extracellular sodium with potassium. Extracellular magnesium and calcium ions significantly reduced the single-channel conductance of HA-SACs. These permeation characteristics and the single-channel conductance of HA-SACs were indistinguishable from those of stretch-activated channels (SACs) previously described in these cells. The following observations are consistent with HA-SACs being a subset of SACs. First, SACs were at times found in cell-attached patches which lacked HA-SACs. Second, the number of channels in a cell-attached patch simultaneously activated by stretch (usually 5-10 and often more) exceeded by far the number simultaneously activated by hyperpolarization (usually one or two).