Physiological and pharmacological changes in the carotid artery pressure-volume curve in situ in rats

Abstract

To measure the mechanical properties of the rat carotid artery as affected by physiological and pharmacological changes. We used an in situ measurement method that did not affect the anatomy, vascularization or innervation of the carotid artery, in normotensive and in spontaneously hypertensive rats (SHR), to evaluate the effects of ageing, hypertension and acute or chronic drug-induced vasorelaxation. The compliance-pressure curve was characterized, under baseline conditions, by two phases, an ascending component in the lower pressure ranges and a descending component in the higher pressure ranges. In younger or older normotensive rats the maximum value of carotid compliance was observed for values of transmural pressure that approximated those of the operating mean blood pressure. For any given value of transmural pressure, carotid compliance was lower in the hypertensive rats, whether younger (3 months) or older (18 months), than in normotensive rats. This finding was particularly striking for transmural pressures below 125 mmHg, and held true even after the administration of potassium cyanide. Consequently, the reduced compliance in the hypertensive rats was attributed predominantly to structural changes in the arterial wall. Moreover, in the hypertensive rats there was a dissociation between the operating arterial blood pressure level and the maximum value of carotid arterial compliance. Thus in young SHR, whereas the operating systemic blood pressure was shifted toward higher values (> 175 mmHg), the maximum value of carotid compliance was observed for pressure levels close to 125 mmHg. This dissociation was more pronounced in older SHR (mean blood pressure 200 mmHg; maximum compliance 75 mmHg). The acute administration of vasodilators or of potassium cyanide induces relaxation of vascular smooth muscle and increases compliance for distension pressures of up to 125 mmHg. Since the drugs had no effects on arterial compliance at higher pressure levels, we conclude that in SHR acute vasorelaxation can increase operating compliance only if blood pressure is reduced. In contrast, chronic treatment with vasodilating agents is able to increase compliance even for higher blood pressure levels. Therefore, we suggest that chronic treatment can improve the elastic properties of the arterial wall through changes in the arterial wall structure.