EFFECTS OF HYPOTHYROIDISM AND HYPERTENSION ON MYOCARDIAL PERFUSION AND VASCULARITY IN RABBITS

We considered the role of thyroid hormones in the growth of the corona ry microvasculature during various levels of afterload. Arterial press ure was increased for 3 mo in hypothyroid and euthyroid rabbits via th e Page (1-kidney, 1-wrap) method. Systolic pressures (mmHg +/- SE) ind icated that the groups could be characterized as follows: euthyroid sh am normotensive (103 +/- 5), euthyroid Page hypertensive (158 +/- 24), hypothyroid Page normotensive (110 +/- 5), and hypothyroid sham hypot ensive (87 +/- 5). The hypothyroid groups were characterized by bradyc ardia. Left ventricular weight-to-body weight ratios were higher in th e Page groups than in the controls. Minimal coronary vascular resistan ce (MCVR) was elevated in the euthyroid Page group but was not affecte d by hypothyroidism. Lumen diameters of the major resistance vessels t ended to be larger in the hypertensive rabbits. Capillary length densi ty was greater in the hypothyroid than in the euthyroid groups. We con clude that 1) the elevated MCVR in the hypertensive rabbits is due mai nly to a failure of the major resistance vessels to increase in size o r number rather than to a decrease in vascular lumen diameter, and 2) hypothyroidism does not affect maximal myocardial perfusion or lumen d iameter of resistance vessels but facilitates capillary growth. The la tter may be due to the presence of bradycardia.