Activation of aβ-adrenergic-sensitive Signal Transduction Pathway by the Secosteroid Hormone 1,25-(OH)2-Vitamin D3in Chick Heart

Abstract

In recent studies we have established that 1α, 25-dihydroxy-vitamin D3[1,25(OH)2D3] rapidly stimulates dihydropyridine-sensitive calcium channel-mediated Ca2+influx in chick cardiac muscle by a non-genomic action which is accompanied by PKA-dependent phosphorylation of a 45 kDa microsomal membrane protein. To investigate the signal transduction pathway activated by 1,25(OH)2D3in heart, we have compared the effects of the secosteroid hormone with those of theβ-adrenergic agonist isoproterenol (IPT) by employing cultured chick embryonic cardiac cells (myocytes) and thin-slice preparations of differentiated adult heart muscle. The increases in45Ca2+uptake and intracellular calcium ([Ca2+]i), cyclic AMP accumulation and changes in microsomal protein phosphorylation evoked by 1,25(OH)2D3could be reproduced by IPT. When combined treatments with the sterol and theβ-adrenergic agonist were performed, no additive stimulation of these parameters was observed, suggesting that a common signal transduction pathway mediates the effects of 1,25(OH)2D3and IPT. The participation of a guanine nucleotide binding protein (G protein) in the 1,25(OH)2D3-induced changes in heart was investigated. AlF4−, an activator of G proteins, and cholera and pertussis toxins, like 1,25(OH)2D3increased45Ca2+uptake by myocytes. AlF4−did not further stimulate the effects of 1,25(OH)2D3thereby showing that a G protein is involved in the hormone action. Moreover, 1,25(OH)2D3potentiated pertussis toxin but was unable to modify choleric toxin-dependent myocyte Ca2+influx. Altogether, these results provide evidence indicating that the non-genomic action of 1,25(OH)2D3on cardiac muscle calcium influx involves modulation of theβ-adrenergic-sensitive adenylyl cyclase/cAMP/PKA pathway coupled to a Gs protein.