Effects of hypothyroidism and hypertension on myocardial perfusion and vascularity in rabbits.

We considered the role of thyroid hormones in the growth of the coronary microvasculature during various levels of afterload. Arterial pressure was increased for 3 mo in hypothyroid and euthyroid rabbits via the Page (1-kidney, 1-wrap) method. Systolic pressures (mmHg +/- SE) indicated that the groups could be characterized as follows: euthyroid sham normotensive (103 +/- 5), euthyroid Page hypertensive (158 +/- 24), hypothyroid Page normotensive (110 +/- 5), and hypothyroid sham hypotensive (87 +/- 5). The hypothyroid groups were characterized by bradycardia. Left ventricular weight-to-body weight ratios were higher in the Page groups than in the controls. Minimal coronary vascular resistance (MCVR) was elevated in the euthyroid Page group but was not affected by hypothyroidism. Lumen diameters of the major resistance vessels tended to be larger in the hypertensive rabbits. Capillary length density was greater in the hypothyroid than in the euthyroid groups. We conclude that 1) the elevated MCVR in the hypertensive rabbits is due mainly to a failure of the major resistance vessels to increase in size or number rather than to a decrease in vascular lumen diameter, and 2) hypothyroidism does not affect maximal myocardial perfusion or lumen diameter of resistance vessels but facilitates capillary growth. The latter may be due to the presence of bradycardia.

Left ventricular structure and performance in middle-aged rats with deoxycorticosterone acetate-salt hypertension.

We designed this study to establish the structural and functional characteristics of the hypertrophied left ventricle of middle-aged rats during mineralocorticoid-salt hypertension. Treatment was initiated at 12 months of age, and the rats were studied at either 13 or 15 months of age (after 1 or 3 months of treatment). All rats were unilaterally nephrectomized. One group received deoxycorticosterone acetate (DOCA) injections (30 mg/kg s.c.) biweekly and 1% NaCl drinking water (DOCA salt), and the other group was injected with the vehicle (sesame seed oil) and given tap water to drink (sham). During the first 4 weeks of DOCA-salt treatment, arterial pressure reached its peak and left ventricular enlargement was mainly due to increases of 47% in cardiocyte cross-sectional area in the middle layer of the left ventricular wall. The last 2 months were characterized by an accelerated endomyocardial growth. Because absolute left ventricular mass did not increase during the last 2 months of treatment, we conclude that cellular hypertrophy was accompanied by a focal loss of cardiocytes. Myocardial hydroxyproline concentration was initially elevated by 37% but normalized by the third month of treatment. Intracellularly, myofibril volume percent was not changed, but mitochondria volume percent declined (13% in the midmyocardium and 15% in the endomyocardium) and sarcoplasmic volume density increased by 25% and 39%, respectively, in these regions. Left ventricular hypertrophy was associated with enhanced peak cardiac and stroke indexes, measured during increased preload, after both 1 and 3 months of DOCA-salt treatment. Acceleration of flow, however, was depressed in the rats with left ventricular hypertrophy.(ABSTRACT TRUNCATED AT 250 WORDS)