Bidirektionale hormonale Modulation spannungsabhängiger Ca2+-Kanle [Bidirectional hormonal modulation of voltage dependent ca2+ channels]

Abstract

The role of guanine nucleotide-binding proteins (G-proteins), acting as transducers between membranous receptors activated by extracellular signals and enzymatic effectors controlling the concentrations of intracellular signal molecules, is well established. G-proteins are also involved in the hormonal modulation of voltage-dependent Ca2+ channels. In various cell types, the increase in intracellular signal molecules via G-protein-coupled receptors causes activation of protein kinases which may stimulate or inhibit voltage-dependent Ca2+ channels. For example, voltage-dependent Ca2+ channels of cardiac and skeletal myocytes are stimulated by cyclic adenosine monophosphate (cAMP)-dependent protein kinase. Other protein kinases, i.e., cyclic guanosine monophosphate (cGMP)-dependent protein kinase and Ca2+/phospholipid-dependent protein kinase C, also appear to be involved in the hormonal modulation of Ca2+ channels. According to this principle, G-proteins exert a distant control of ion channel activity. In addition, there appears to exist another mechanism which does not involve intracellular signal molecules or protein kinases stimulated by intracellular signal molecules. The only signal transduction components identified so far include receptors, G-protein and Ca2+ channels. Ca2+ channel modulations following this apparently membrane-confined mechanism have been described to occur in neuronal, endocrine and cardiac cells. Hormonal inhibition of Ca2+ channels in neuronal and endocrine cells is mediated by a pertussis-toxin-sensitive G-protein, possibly G0. The G-protein involved in the hormonal stimulation of Ca2+ channels in adrenocortical and pituitary cells may represent a pertussis-toxin-sensitive G-protein of the Gi-type. The choleratoxin-sensitive G-protein, Gs, may stimulate cardiac Ca2+ channels without the involvement of a cAMP-dependent intermediate step