Effect of thyroid status on thin-filament Ca2+ regulation and expression of troponin I in perinatal and adult rat hearts.

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Abstract
There is evidence for the existence of developmental changes in expression of troponin I (TNI) in cardiac thin filaments; however, regulation of TNI expression has not been described. We tested whether thyroid state affects expression of TNI using neonatal and adult rats made hypothyroid by treatment with 6-n-propyl-2-thiouracil. Polyacrylamide gels of myofibrils from hearts of 7-, 14-, 21-, and 28-day-old animals indicated that both euthyroid and hypothyroid rats display a developmental shift toward the adult form of TNI. However, hypothyroid rats displayed a lower percentage of adult TNI at each age studied. When adult rats were made hypothyroid, the proportion of adult TNI decreased slightly. Thin-filament activity was determined from measurements of the effect of acidic pH on calcium activation of myofibrillar ATPase activity. Sensitivity to acidic pH was measured by the magnitude of shift in pCa50 (-log of half-maximally activating molar Ca2+) between pH 7.0 and 6.5. Euthyroid rats displayed developmental increases in pH sensitivity. At 7, 14, and 28 days of development, shifts in pCa50 were 0.11, 0.38, and 0.43 units, respectively. Hypothyroid rats displayed less pH sensitivity with pCa50 shifts of 0.07, 0.21, and 0.15 units at 7, 14, and 28 days of development. Adult hypothyroid rats displayed a 0.38-unit shift in pCa50, whereas euthyroid adults displayed a 0.44-unit shift. Our results indicate that pH sensitivity and expression of cardiac TNI are influenced by developmental stage and hormonal status.