Pertussis toxin-sensitive Gi-proteins and intracellular calcium sensitivity of vasoconstriction in the intact rat tail artery

1 We studied the involvement of pertussis toxin (PTX)-sensitive C-proteins in the sensitivity of arterial constriction to intracellular calcium ([Ca2+](i)) mobilization. 2 Vasoconstriction was measured in vitro in perfused, de-endothelialized rat tail arteries loaed with the calcium-sensitive dye, fura-2 and treated or not with PTX (30-1000 ng ml(-1)). Arteries were stimulated with noradrenaline (NA, 0.1-100 muM) or KCI (15-120 mM). 3 KCI elicited a smaller vasoconstrictor response (E-max = 94 +/- 8 mmHg) than NA (E-max=198+/-9 mmHg) although [Ca2+](i) mobilization was similar (E-max=123+/-8 and 135+/-7 nM For KCl and NA, respectively). PTX (1000 ng ml(-1)) had no effect on [Ca2+](i) mobilization but lowered NA- (but not KCl-) induced vasoconstriction (E-max=118+/-7 mmHg). 4 G(i,o)-proteins were revealed by immunoblotting with anti-G(i alpha) and anti-G(o alpha) antibodies in membranes prepared from de-endothelialized tail arteries. [alpha P-32]-ADP-ribosylation of G-proteins by PTX (1000 ng ml(-1)) was demonstrated in the intact rat tail artery (pixels in the absence of PTX: 3150, presence: 25053). 5 In conclusion, we suggest that smooth muscle cells possess a PTX-sensitive G(i)-protein-mediated intracellular pathway which amplifies [Ca2+](i) sensitivity of contraction in the presence of agonists such as NA.