Effects of Altered Smooth Muscle Tone on Aortic Diameter and Aortic Baroreceptor Activity in Anesthetized Rabbits

The possible role of aortic smooth muscle in the regulation of aortic diameter and activity in the aortic baroreceptors was investigated in anesthetized rabbits. Aortic diameter was measured with ultrasonic technique within the intact thorax, and activity recorded in the whole left aortic nerve was quantified by rectification and integration. Blood pressure was changed by bleeding and reinfusion. At comparable blood pressures, norepinephrine (1 to 6 μ/kg.min) produced a 1% to 10% constriction of the aorta, without affecting the pressure-strain elastic modulus (EP). After phenoxybenzamine, maximum increases in diameter and Ep were 17% and 280%, respectively. When related to diastolic diameter, however, Ep was mostly increased by norepinephrine and reduced by phenoxybenzamine. Baroreceptor activity at comparable pressures remained normal during infusion of norepinephrine, but increased after phenoxybenzamine. However, when compared at equal diastolic diameters, the sensitivity of the receptors to aortic distention was found to be increased by norepinephrine and unaltered by phenoxybenzamine. Increased smooth muscle tone therefore served to maintain a normal relationship between blood pressure and baroreceptor activity, in spite of the simultaneous reduction in aortic diameter. Reduction of smooth muscle tone markedly increased activity in the aortic baroreceptors, due to dilatation of the receptor area.