Objective: To test the hypothesis that hypoxia activates ATP-sensitive K+ (KATP) channels in cremasteric arteriolar muscle cells, resulting in membrane hyperpolarization and inhibition of norepinephrine-induced contraction.
Methods: Arteriolar muscle cells were isolated enzymatically from second- and third-order arterioles that were surgically removed from hamster cremaster muscles. The effects of hypoxia (PO2 = 12-15 mm Hg) were then examined on norepinephrine-induced contraction, membrane currents, and membrane potential in these cells at room temperature. Whole-cell currents and membrane potential were recorded using the perforated patch technique.
Results: Hypoxia (12-15 mm Hg PO2) reversibly inhibited norepinephrine-induced contraction to 52 +/- 6% of the response in normoxic solutions (156 mm Hg, n = 12 digests, p < 0.05). These effects of hypoxia could be prevented by superfusion of the cells with either solutions containing the KATP channel antagonist glibenclamide (1 microM) or solutions containing 35 mM K+ to reduce the electrochemical gradient for K+ diffusion. Cromakalim, an activator of KATP channels, also inhibited norepinephrine-induced contraction to a similar extent as hypoxia, and in a glibenclamide and 35 mM K(+)-sensitive manner. These results are consistent with the KATP channel hypothesis. In contrast, hypoxia had no effect on estimated whole-cell membrane conductance between -40 and -90 mV in voltage-clamp experiments; on holding current measured at -60 mV in cells superfused with 143 mM K+ under voltage-clamp conditions; or on membrane potential in current-clamp experiments, despite positive effects of cromakalim in all three protocols. These electrophysiological data lead to rejection of the hypothesis that hypoxia activates KATP channels.
Conclusions: Hypoxia inhibits norepinephrine-induced contraction of cremasteric arteriolar muscle cells by a mechanism that does not involve KATP channels. It is speculated that the inhibitory effects of glibenclamide and 35 mM K+ on the effects of hypoxia on contraction resulted from depolarization induced by these treatments rather than specific inhibition of KATP channels.