Hypoxia Inhibits Contraction but Not Calcium Channel Currents or Changes in Intracellular Calcium in Arteriolar Muscle Cells

Objective: We tested the hypothesis that hypoxia inhibits currents through L‐type Ca 2+ channels and inhibits norepinephrine‐induced rises in intracellular Ca 2+ in cremasteric arteriolar muscle cells, thus accounting for the inhibitory effect of hypoxia on norepinephrine‐induced contraction of these cells. Methods: Single smooth muscle cells were enzymatically isolated from second‐order and third‐order arterioles from hamster cremaster muscles. The effects of hypoxia (partial pressure of oxygen: 10–15 mm Hg) were examined on Ba 2+ (10 mM) currents through L‐type Ca 2+ channels by use of the perforated patch clamp technique. Also, the effect of hypoxia on norepinephrine‐induced calcium changes was studied using Fura 2 microfluorimetry. Results: Hypoxia inhibited the norepinephrine‐induced (10 µM) contraction of single arteriolar muscle cells by 32.9 ± 5.6% (mean ± SE, n = 4). However, hypoxia had no significant effect on whole‐cell currents through L‐type Ca 2+ channels: the peak current densities measured at +20 mV were −3.83 ± 0.40 pA/pF before hypoxia and −3.97 ± 0.36 pA/pF during hypoxia ( n = 15; p > 0.05). In addition, hypoxia did not inhibit Ca 2+ transients in arteriolar muscle cells elicited by 10 µM norepinephrine. Instead, hypoxia increased basal Ca 2+ (13.8 ± 3.2%) and augmented peak Ca 2+ levels (29.4 ± 7.3%) and steady‐state Ca 2+ levels (15.2 ± 5.4%) elicited by 10 µM norepinephrine ( n = 21; p < 0.05). Conclusions: These data indicate that hypoxia inhibits norepinephrine‐induced contraction of single cremasteric arteriolar muscle cells by a mechanism that involves neither L‐type Ca 2+ channels nor norepinephrine‐induced Ca 2+ mobilization. Instead, our findings suggest that hypoxia must inhibit norepinephrineinduced contraction by affecting a component of the signaling pathway that lies downstream from the increases in Ca 2+ produced by this neurotransmitter.