Carbon monoxide-induced cardiac hypertrophy is not reduced by alpha- or beta-blockade in the rat

The stimulus for carbon monoxide-induced cardiac hypertrophy was investigated. Two experiments were carried out in which adult male Sprague-Dawley rats were exposed continuously to 700 ppm CO for 30 days (CO) or inhaled room air (Air). In each experiment, 2/3s of the rats received either the beta-1-adrenergic blocker, atenolol, or the alpha-1 adrenergic blocking agent, prazosin, in the food daily, at low and high doses. Systolic blood pressure (SBP) was significantly lowered (20-25 mmHg) by CO alone. Atenolol alone lowered SBP, but only at the high dose. Low dose and particularly high dose atenolol, lowered SBP even more in the CO rats. Prazosin lowered SBP, particularly at the high dose and further lowered SBP in the CO rats. Heart rate was significantly lowered by atenolol and prazosin alone at both doses in the Air rats. Heart rate remained the same or was slightly elevated by CO exposure. Heart rate in the presence of CO was significantly depressed by prazosin, but not by atenolol. Carbon monoxide alone resulted in 30-43% and 18-25% weight increases in right ventricle free-wall (RV) and left ventricle + septum (LV+S), respectively, relative to untreated controls. Neither low nor high dose prazosin significantly decreased RV and LV+S weights in the CO rats. Low dose atenolol failed to alter RV and LV+S weights in the CO rats; however, high dose atenolol, significantly (P < 0.01) increased RV weight in the CO rats. Right ventricle weight was positively correlated with SBP lowering by CO and/or atenolol, or prazosin. Carbon monoxide exposure increased lung/body weight ratio; atenolol, but not prazosin, attenuated this effect. Hematocrit increased from 50% in the Air to 77% in the CO rats; it was unaltered by prazosin or atenolol treatment. Thus, CO-induced cardiac hypertrophy develops in spite of lowered SBP (i.e. lowered LV afterload), and the blockade of either alpha or beta-1 receptors. It is suggested that the increased ventricular preload caused by atenolol and prazosin is directly responsible for the cardiac hypertrophy, regardless of the ameliorating effects of decreased inotropicity and heart rate produced by the adrenergic blocking agents. The results suggest the potentially powerful role of enhanced preload in driving myocardial hypertrophy.