SHORT-TERM AND LONG-TERM EFFECTS OF NEONATAL HYPOTHYROIDISM AND HYPERTHYROIDISM ON CORONARY ARTERIOLES IN RAT

Neonatal hypo- and hyperthyroid effects on coronary arteriolar geometr y were examined in newborn male Sprague-Dawley rats treated for 12 or 28 days with either triiodothyronine or propylthiouracil. Long-term ef fects were assessed in weaned rats 52 days after stopping treatment. I nfluence of birth neonatal conditions was more pronounced after 28 day s. Neonatal hyperthyroidism induced cardiac hypertrophy; neonatal hypo thyroidism attenuated cardiac growth. Hyperthyroid rats had similar ar teriolar and capillary numerical densities and arteriolar length densi ty but significantly greater (P < 0.05) total arteriolar length than c ontrol. Hypothyroid rats had similar arteriolar numerical. and length densities, greater capillary numerical density (P < 0.05), but markedl y lower total arteriolar length (P < 0.01) than control. Results sugge st that neonatal hyperthyroidism stimulates arteriolar and capillary g rowth, whereas neonatal hypothyroidism attenuates arteriolar but not c apillary growth. After cessation of treatment, total arteriolar length in previously hyperthyroid rats did not change despite increased card iac mass, whereas previously hypothyroid rats demonstrated marked incr eases in both cardiac mass and total arteriolar length (P < 0.01). The se results indicate a lasting inhibitory effect of early hyperthyroidi sm on subsequent arteriolar growth.