Increased Ca2+ sensitivity as a key mechanism of PKC-induced constriction in pressurized cerebral arteries

The effects of activating protein kinase C (PKC) with indolactam V (Indo-V) and 1,2-dioctanoyl-sn-glycerol (DOG) on smooth muscle intracellular Ca(2+) concentrations ([Ca(2+)](i)) and arterial diameter were determined using ratiometric Ca(2+) imaging and video edge detection of pressurized rat posterior cerebral arteries. Elevation of intraluminal pressure from 10 to 60 mmHg resulted in an increase in [Ca(2+)](i) from 74 +/- to 219 +/- nM and myogenic constriction. Application of Indo-V (0.01-3 mu M) or DOG; (0.1-30 mu M) induced constriction and decreased [Ca(2+)](i) to 140 +/- 11 and 127 +/- 12 nM, respectively, at the highest concentrations used. In the presence of Indo-V, the dihydrapyridine Ca(2+)-channel blocker nisoldipine produced nearly maximum dilation and decreased [Ca(2+)](i) to 97 +/- 7 nM. In alpha-toxin-permeabilized arteries, the constrictor effects of Indo-V and DOG were not observed in the absence of Ca(2+). Both PKC activators significantly increased the degree of constriction of permeabilized arteries at different [Ca(2+)](i). We conclude that I) Indo-V- or DOG-induced constriction of pressurized arteries requires Ca(2+) influx through voltage-dependent Ca(2+) channels, and 2) PKC-induced constriction of pressurized rat cerebral arteries is associated with a decrease in [Ca(2+)](i), suggesting an increase in the Ca(2+) sensitivity of the contractile process.