Differences in pH sensitivity of tension generation between developing and adult cardiac myofilaments, which contain the same isoform of troponin C (TnC), have been proposed to be due to troponin I (TnI) isoform switching from the slow skeletal (ss) to cardiac (c) TnI isoforms (21). We investigated the effects of acidic pH on Ca(2+)-activation of force in chemically skinned preparations of adult rat trabeculae and single soleus fibers that also share the same TnC isoform. Compared with the soleus fibers, trabeculae demonstrated a greater suppression of tension and a rightward shift in pCa50 (-log half-maximally activating molar Ca2+ concentration) when pH was decreased from 7.0 to 6.2. The pH-induced shift in pCa50 in soleus fibers did not change with sarcomere length. Troponin subunit interactions were also investigated, using cardiac troponin C (cTnCIA) labeled with a fluorescent probe, 2-(4'-iodoacetamidoanilino)-naphthalene-6-sulfonic acid. Under acidic conditions, cTnCIA demonstrated a decrease in Ca(2+)-affinity. This decrease was amplified both in the binary complex cTnCIA-cTnI and in the complex cTnCIA-cTnI-cTnT-tropomyosin to the same extent. In contrast, substitution of ssTnI for cTnI in these complexes produced the same decrease in Ca2+ affinity in response to acidic pH as cTnCIA alone. These results support our hypothesis that differential effects of pH on tension generation and Ca2+ sensitivity between soleus fibers and trabeculae are due to the presence of different isoforms of TnI.

Bupharesponsiveness, was demonstrated in ovariectomized hearts. In parallel measurements of SERCA activity in SR-enriched membrane preparations from ovariectomized hearts, a suppressed maximum SERCA activity with a leftward shift in the relationship between pCa (-log molar free Ca 2ϩ concentration) and SERCA activity was also detected. A significant downregulation of SERCA proteins and reduction in the SERCA mRNA level were observed in association with suppressed maximum SERCA activity. While there were no changes in total phospholamban and phosphorylated Ser 16 phospholamban levels, a decrease in phosphorylated Thr 17 phospholamban as well as an increase in the suprainhibitory, monomeric form of phospholamban stoichiometry was found. Estrogen and progesterone supplementations were equally effective in preventing changes in ovariectomized hearts. Our data showed for the first time that female sex hormones played an important role in the regulation of the cardiac SR Ca 2ϩ uptake. Under hormone-deficient conditions, there was an adaptive response of SERCA that escaped the regulatory effect of phospholamban. sarcoplasmic reticulum Ca 2ϩ -adenosine 5Ј-triphosphate activity; phospholamban; phosphorylation THE CARDIOREGULATORY ROLE of female sex hormones in contractile activity has been raised in many studies (23, 24, 28, 34 -36). Expression of both estrogen and progesterone receptors in myocytes indicated a direct effect of the hormones on myocardial function (14, 18). Studies of postmenopausal women using echocardiography have demonstrated significant decreases in cardiac contractility compared with those of premenopausal women (23,24). Reductions in stroke work and fractional shortening have also been demonstrated in studies using whole heart preparations of ovariectomized rats (28). In addition, suppression of both myosin ATPase activity and maximum myofibrillar ATPase activity with an upregulation of ␤-myosin heavy chain in ovariectomized rat hearts was reported (28,34,35). All evidence indicated important roles of female sex hormones in myocardial activation. The striking finding was an increase in the Ca 2ϩ responsiveness of cardiac myofilament activation with almost the same magnitude in ovariectomized hearts as that in cardiomyopathic failing hearts (17,34,35,38,39). The information thus implied a potential of cardiomyopathic induction by female sex hormone deficiency. It was, however, not known how myofilament Ca 2ϩ hypersensitivity was induced in the hormone-deficient heart. A reasonable hypothesis was that this effect may be an adaptive response of the heart to changes in the intracellular free Ca 2ϩ

The hypothesis that ovarian sex hormone deficiency affects cardiac myofilament activation was tested. Chemically skinned ventricular trabeculae and single soleus muscle fibers were prepared from 10- and 14-wk ovariectomized and control rats. Tension-pCa (−log [Ca2+]) relations of left ventricular trabeculae and soleus fibers were compared to test whether thin filament proteins are potential sites of modulated activation. Trabeculae from ovariectomized rats exhibited a significant increase in Ca2+ sensitivity with no change in maximal tension-generating ability. In contrast, soleus fibers demonstrated no shift in Ca2+ sensitivity but generated significantly less maximal tension. No changes in thin filament protein isoform expression or loss of thin filament proteins were apparent in the trabeculae or soleus fibers from ovariectomized rats. Although not directly tested, our results are consistent with a possible modulation of regulatory proteins (e.g., cardiac troponin I) to account for the observed change in myofilament responsiveness of hearts from ovariectomized rats. Other possible mechanisms for the altered myocardial Ca2+ sensitivity after ovariectomy are discussed.

No abstract