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Mechanism of hyperthyroidism-induced modulation of the L-type Ca2+ current in guinea pig ventricular myocytes

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Abstract

The positive inotropic effects of thyroid hormone in the heart, increased force and velocity of contraction have been mostly attributed to modulation of myosin ATPase isoenzymes (V1, V2 and V3), and sarcoplasmic reticulum Ca2+ pumping activity. In addition, we have suggested that the effects on ventricular contraction result from a thyroid hormone-induced increase in L-type Ca2+ current (I Ca, L). Due to the central role of I Ca, L in excitation-contraction coupling, we studied mechanisms whereby thyroid hormone augments this current. Since thyroid hormone modulates adenylate cyclase activity in various tissues, we tested the hypothesis that the hormone activates adenylate cyclase, leading to increased cyclic adenosine monophosphate (cAMP) levels, protein kinase A activation, Ca2+ channel phosphorylation and increased I Ca, L. We therefore stimulated or inhibited different sites along the “adenylate cyclase cascade”, and measured I Ca, L and isometric twitch in ventricular myocytes and papillary muscles from euthyroid and hyperthyroid guinea pigs. Our major findings were as follows. In euthyroid myocytes, 0.1 μM isoproterenol (Iso) increased I Ca, L (at V M=0 mV) from −7.04±0.72 to −22.26±1.88 pA/pF, P<0.05, while in hyperthyroid myocytes (I Ca, L=-21.48±2.94 pA/pF), Iso was ineffective. In euthyroid myocytes, intracellular application of cAMP (50 μM) was as potent as Iso, but ineffective in hyperthyroid myocytes. In hyperthyroid myocytes, a protein kinase A inhibitor (2 μM) lowered I Ca, L from −26.82±1.54 to -10.17±1.70 pApF (P<0.05), but had no effect in euthyroid myocytes. In hyperthyroid myocytes, acetylcholine (ACh) (1 μM) decreased I Ca, L from −26.86±1.49 to −18.33±1.25 pA/pF (P<0.05), while in euthyroid myocytes ACh decreased ICa, L from −6.80±0.61 to −6.00±0.39 pA/pF (NS). Accordingly, in hyperthyroid papillary muscles, ACh decreased twitch tension by 36.4±2.8%, but in euthyroid preparations only by 9.4±5.1% (P<0.05). These findings suggest that thyroid-hormone-induced increase in I Ca, L contributing to positive inotropy, is mediated by activation of the adenylate cyclase cascade.

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Authors and Affiliations

  1. Department of Physiology and Biophysics, Bruce Rappaport Faculty of Medicine, Technion, P.O. Box 9697, 31096, Haifa, Israel

    Sela Mager, Yoram Palti & Ofer Binah

  2. The Rappaport Institute for Research in the Medical Sciences, P.O. Box 9697, 31096, Haifa, Israel

    Sela Mager, Yoram Palti & Ofer Binah

Authors
  1. Sela Mager
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  2. Yoram Palti
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  3. Ofer Binah
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Mager, S., Palti, Y. & Binah, O. Mechanism of hyperthyroidism-induced modulation of the L-type Ca2+ current in guinea pig ventricular myocytes. Pflügers Arch 421, 425–430 (1992). https://doi.org/10.1007/BF00370252

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  • DOI: https://doi.org/10.1007/BF00370252

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