DEMONSTRATION OF THE ALPHA-VASOCONSTRICTION-LOWERING EFFECT OF CALCIUM IN THE PITHED RAT

The mechanism by which calcium administration causes hypotension still remains to be explained. In this paper we have studied the alteration of the pressor effect caused by alpha1- and alpha2-adrenoceptor stimu lation in the pithed rat after increasing the calcium plasma levels of the rats with an intraperitoneal administration of CaCl2. We compared the vasoconstrictor effect of the selective alpha1-adrenoceptor agoni st methoxamine (10-3,000 mug/kg) and the pressor effect of the selecti ve alpha2-adrenoceptor agonist B-HT 920 (1-1,000 mug/kg) in control ra ts (1.24 +/- 0.01 mmol/l ionic plasma calcium) and rats treated intrap eritoneally with CaCl2 that had 1.89 +/- 0.04 or 3.03 +/- 0.11 mmol/l ionic plasma calcium. The pressor effect of methoxamine was noticeably less after both of the increases in ionic plasma calcium, and the pre ssor effect of B-HT 920 was also significantly reduced when the increa se in calcium levels was more marked. We have also shown that the admi nistration of CaCl2 (8.8 mg Ca/100 g b.w. i.p.) did not alter the basa l arterial blood pressure in the pithed rat; however, it caused an imp ortant decrease in blood pressure when administered 20 min after begin ning an infusion of methoxamine (50 mug/min i.v.). The results indicat ed that the blood-pressure-lowering effect of acute calcium administra tion is at least in part due to peripheral mechanisms which include a reduction in the pressor responses caused by the alpha1 and alpha2 vas cular receptor stimulation.