Role of the autonomic nervous system in the pressor response to calcium in conscious dogs.

Abstract

The effects of acute hypercalcaemia on arterial pressure and vascular tone have been poorly understood. We analysed the effect of a bolus of calcium chloride (15 mg . kg-1 iv) on arterial pressure, total peripheral resistance, and left ventricular function in 25 conscious dogs studied with or without pharmacological autonomic blockade. Without autonomic blocking drugs, the maximum response to calcium included increases of +139.4 kPa . s-1 (+1046 mmHg . s-1) in maximum rate of change of left ventricular pressure, +2.2 cm3 in stroke volume, +2.6 kPa (+ 19.2 mmHg) in aortic systolic pressure, and +0.6 kPa (+4.4 mmHg) in mean aortic pressure, but total peripheral resistance was unchanged. During beta-adrenergic blockade with propranolol, calcium again increased maximum rate of change of pressure and stroke volume, increased mean aortic pressure (2.2 kPa [+16.5 mmHg]), and increased resistance by 35%. When calcium was given during alpha-adrenergic blockade with phenoxybenzamine or phentolamine, mean aortic pressure did not rise, and resistance fell by 19%. The calcium-induced rise in resistance during beta-adrenergic blockade was abolished by surgical adrenalectomy. We conclude that excess extracellular calcium ion may influence vascular resistance by increasing autonomic nervous system excitation of alpha- and beta-adrenergic vascular receptors. A major mechanism by which the sympathetic nervous system effects occur is through increased release of catecholamines from the adrenal medulla.