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 alpha 1- and alpha 2-adrenoceptor stimulation 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 alpha 1-adrenoceptor agonist methoxamine (10-3,000 micrograms/kg) and the pressor effect of the selective alpha 2-adrenoceptor agonist B-HT 920 (1-1,000 micrograms/kg) in control rats (1.24 +/- 0.01 mmol/l ionic plasma calcium) and rats treated intraperitoneally 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 pressor effect of B-HT 920 was also significantly reduced when the increase in calcium levels was more marked. We have also shown that administration of CaCl2 (8.8 mg Ca/100 g b.w. i.p.) did not alter the basal arterial blood pressure in the pitched rat; however, it caused an important decrease in blood pressure when administered 20 min after beginning an infusion of methoxamine (50 micrograms/min i.v.). The results indicated that the blood-pressure-lowering effect of acute calcium administration is at least in part due to peripheral mechanisms which include a reduction in the pressor responses caused by the alpha 1 and alpha 2 vascular receptor stimulation.