Interactions between aortic pressure and inotropic responses in the carotid sinus baroreflex of the rabbit

The isolated left carotid sinus of rabbit was stimulated with 20 kPa pressure steps. By means of an intraaortic balloon catheter the arterial blood pressure could be either fixed against reflex changes (pressure clamp) or adjusted to any value desired. The passive pressure dependence of the left ventricular systolic pressure LVSP is observed to be linear, and the enddiastolic pressure LVEDP is hardly affected. The LV contractility index CI = (d log P/dt)max has a nonlinear dependence on the aortic pressure. Its slope, the so-called coefficient of pressure-induced inotropy CPI, is high in the lower pressure range and vice versa. In the paced heart inotropic reflex responses to 20 kPa intrasinus pressure steps, expressed in percent attenuation of the CI, were firstly elicited at freely variable arterial pressure and, thereupon, at pressure clamp conditions. In the latter case the inotropic effects are significantly lower (14.5%) than at free running pressure (34.6%). A mean of CPI = 2.32% . kPa-1 is obtained from the inotropic effect differences and the concomitant pressure drops. When subdividing the effects with respect to initial diastolic pressure, we could obtain a CPI = 3.7% . kPa-1 in the pressure range below 14.5 kPa, and a CPI = 1.53% . kPa-1 for higher pressures. The primary inotropic effects at pressure clamp conditions do not depend on the arterial pressure.