Changes in Thyroid State Affect pHiand Nai+Homeostasis in Rat Ventricular Myocytes

We have tested the hypothesis that thyroid state may influence both the flow of cellular Ca2+and the myofilament response to Ca2+by effects on intracellular pH (pHi) and Na+(Nai+). Single cardiac myocytes isolated from hypothyroid, euthyroid and hyperthyroid animals were loaded with fura-2/AM (Cai2+probe), BCECF/AM (pHiprobe) or SBFI/AM (Nai+probe). Compared with hypothyroid animals, myocytes isolated from hyperthyroid rat hearts demonstrated a significant: (1) increase in extent of shortening; (2) decrease in the time to peak contraction; (3) increase in the peak amplitude of the fura-2 fluorescence ratio; (4) decrease in pHi(ΔpHi=0.19±0.05); and (5) increase in Nai+(ΔNai+=2.88±0.55 m ). We have also compared pHiin Langendorff perfused hypo- and hyperthyroid rat hearts using NMR. We have found that hyperthyroid hearts are 0.15±0.03 pH units more acidic than hypothyroid hearts. Analysis of mRNA levels demonstrated that hyperthyroidism increased expression of both the Na+/Ca2+exchanger and Na+/H+antiporter, and decreased expression of Na+channel mRNAs. These changes appear partially responsible for the observed changes in Nai+and pHi. Our results provide the first evidence that changes in cardiac contractility associated with altered thyroid state not only involve effects on Ca2+, but may also involve changes in the response of the myofilaments to Cai2+mediated by altered pHiand Nai+.