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.
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