Abstract
Objective:We tested the hypothesis that hypoxia inhibits currents through L-type Ca2+channels and inhibits norepinephrine-induced rises in intracellular Ca2+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 Ba2+(10 mM) currents through L-type Ca2+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 Ca2+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 Ca2+transients in arteriolar muscle cells elicited by 10 µM norepinephrine. Instead, hypoxia increased basal Ca2+(13.8 ± 3.2%) and augmented peak Ca2+levels (29.4 ± 7.3%) and steady-state Ca2+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 Ca2+channels nor norepinephrine-induced Ca2+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 Ca2+produced by this neurotransmitter. Microcirculation(2003) 10,133–141. doi:10.1038/sj/mn.7800178