Characterization and function of Ca2+-activated K+ channels in arteriolar muscle cells

We examined the functional role of large-conductance Ca2+-activated K+ (K-Ca) channels in the hamster cremasteric microcirculation by intravital videomicroscopy and characterized the single-channel properties of these channels in inside-out patches of membrane from enzymatically isolated cremasteric arteriolar muscle cells. In second-order (39 +/- 1 mu m, n = 8) and third-order (19 +/- 2 mu m, n = 8) cremasteric arterioles with substantial resting tone, superfusion with the K-Ca channel antagonists tetraethylammonium (TEA, 1 mM) or iberiotoxin (IBTX, 100 nM) had no significant effect on resting diameters (P > 0.05). However, TEA potentiated O-2-induced arteriolar constriction in vivo, and IBTX enhanced norepinephrine-induced contraction of cremasteric arteriolar muscle cells in vitro. Patch-clamp studies revealed unitary K+-selective and IBTX-sensitive currents with a single-channel conductance of 240 +/- 2 pS between -60 and 60 mV (n = 7 patches) in a symmetrical 140 mM K+ gradient. The free Ca2+ concentration ([Ca2+]) for half-maximal channel activation was 44 +/- 3, 20 +/- 1, 6 +/- 0.4, and 3 +/- 0.5 mu M at membrane potentials of -60, -30, +30, and +60 mV, respectively (n = 5), with a Hill coefficient of 1.9 +/- 0.2. Channel activity increased e-fold for a 16 +/- 1 mV (n = 6) depolarization. The plot of log[Ca2+] vs. voltage for half-maximal activation (V-1/2) was linear (r(2) = 0.9843, n = 6); the change in V-1/2 for a 10-fold change in [Ca2+] was 84 +/- 5 mV, and the [Ca2+] for half-maximal activation at 0 mV (Ca-0; the Ca2+ set point) was 9 mu M. Thus, in vivo, K-Ca channels are silent in cremasteric arterioles at rest but can be recruited during vasoconstriction. We propose that the high Ca-0 is responsible for the apparent lack of activity of these channels in resting cremasteric arterioles, and we suggest that this may result from expression of unique K-Ca channels in the microcirculation.