[Effect of thyroid hormone on the beta-receptor of cardiac myocytes isolated from rat ventricle]

Hypoxia-induced adaptational shift in MHC-beta isoform expression in rat ventricles

We investigated whether the shift of cardiac myosin heavy chain (MHC) isoform observed during exposure to hypoxia is secondary to hypertrophy, or whether it is directly related to the hypoxic stress. Twelve male Wistar-Kyoto rats, 14 weeks old, were randomly assigned to two groups: sea-level control group (CO) and hypoxia group (HX). The CO group was housed 4 weeks at 1,011 hPa, and the HX group was housed for 4 weeks at 701 hPa. The expression of MHC-beta was significantly increased (600%) in the HX group as compared to the CO group in the right ventricle (p < 0.01). An increased ventricular mass induced by hypoxic exposure was associated with an increased expression of MHC-beta in the right ventricle (p < 0.05). In the left ventricle, the MHC-b expression was significantly increased (295%) in the HX group as compared to the CO group without ventricular hypertrophy (p < 0.01). No differences were observed in the adenylyl cyclase activity or in the phosphodiesterase activities in both ventricles between the CO and HX groups (p > 0.05). Oxidative enzymatic activities (citrate synthase and three-hydroxyacyl-CoA dehydrogenase) were unchanged in both ventricles following 4 weeks of hypoxia (p > 0.05). These findings suggest that, besides cardiac hypertrophy, the hypoxia-induced adaptational change to the MHC-b isoform may be mediated through a specific mechanism related to the stress of hypoxia.

Angiotensin-converting enzyme inhibitor up-regulates cardiac beta-receptors in cultured neonatal rat myocytes

In patients with congestive heart failure (CHF), beta-receptor up-regulation is regarded as one of the mechanisms leading to improved function and prognosis. To clarify whether beta-receptor up-regulation is involved in the mechanisms underlying the beneficial effects of angiotensin-converting enzyme (ACE) inhibitors, we investigated the actions of ACE inhibitors and an angiotensin II type 1 receptor (AT1) antagonist on beta-receptors of neonatal rat cultured cardiac myocytes. Angiotensin II (A-II) increased the spontaneous beating frequency of the cells, and the effect was completely antagonized by the AT1 antagonist CV-11974. Under control conditions, beta-receptor density (Bmax) and affinity (Kd) were measured by radiobinding assay with the hydrophilic ligand [3H]CGP-12177, and were 103 +/- 11 fmol/mg protein and 3.4 +/- 0.4 nmol/L, respectively. Captopril increased the beta-receptor density of myocytes and augmented the response to isoproterenol. Bmax was increased by 34% after 24 h treatment with 10(-6) mol/L captopril. CV-3480, and ACE inhibitor that contains no sulfhydryl group, but neither A-II nor the AT1 antagonist, also up-regulated beta-receptors. The results suggest that beta-receptor up-regulation contributes at least partly to the beneficial cardiac effects of ACE inhibitors in patients with CHF. ACE inhibitors and AT1 antagonists seem to play different roles in clinical practice.