Mechanical effects of vasoactive drugs on carotid sinus

Carotid sinus diameter (CSD) is influenced by changes in sympathetic tone and vasoactive agents. This study was designed to determine which mechanical properties of the carotid sinus region were influenced by infusing vasoconstrictors (epinephrine, 4.56 X 10(-6) M, and phenylephrine, 9.85 X 10(-5) M) and a vasodilator (nitroprusside, 1.68 X 10(-4) M). CSD, carotid sinus length (CSL), pressure (CSP), and compliance (CSC), and arterial pressure were all recorded simultaneously from the isolated constant-flow-perfused carotid sinus region of 11 anesthetized dogs (35 mg/kg pentobarbital sodium) before and after drug perfusion. CSC was measured by a method previously described in which 13 microliters of perfusate is injected into the segment in a step-like manner and the resultant step change in pressure recorded. The compliance of the vessel segment is read on-line after a calibration procedure. CSD and CSL were measured using sonomicrometer length gauges positioned across and along the length of the carotid sinus segment. At a CSP of 99.9 +/- 0.6 (SE) mmHg, CSD, CSL, and CSC were 8.50 +/- 0.44 mm, 9.44 +/- 0.84 mm, and 0.46 +/- 0.05 microliter/mmHg, respectively. Decreasing CSP to 50 mmHg significantly reduced CSD and CSL and increased CSC. Increasing CSP to 150 mmHg produced opposite results. Vasoconstrictor drug infusion significantly decreased and vasodilator drug infusion significantly increased both CSD and CSL, producing parallel shifts in the CSP-CSD and -CSL curves toward and away from the pressure axis. The shift to new pressure-volume curves resulted in no change in CSC in response to the vasoactive agents.