Abstract
This work aimed to study the relationship between agonist-induced changes in cytosolic free calcium levels, protein kinase C (PKC) activity and intracellular pH in isolated liver cells. We observed that, like alpha1-adrenergic agonists, the Ca2+-mobilizing vasoactive peptides vasopressin and angiotensin II produced an extracellular-Na+-dependent, 5-(N-ethyl-N-isopropyl)amiloride-sensitive, intracellular alkalinization, indicative of Na+/H+ antiporter activation. Blocking the agonist-induced increase in the intracellular Ca2+ concentration using the calcium chelator bis-(o-aminophenoxy)ethane-N,N,N', N'-tetra-acetic acid (BAPTA) prevented all types of receptor-mediated intracellular alkalinization. Thus activation of the Na+/H+ exchanger by either alpha1-adrenergic agonists or vasoactive peptides relies on the mobilization of intracellular Ca2+. In contrast, only the alpha1-adrenergic-agonist-induced alkalinization was dependent on extracellular Ca2+. Even though alpha1-adrenergic as well as vasoactive peptide agonists stimulated protein kinase C (PKC) activity in isolated liver cells, only the alpha1-adrenoreceptor-mediated intracellular alkalinization was dependent on PKC. According to these observations, Ca2+-mobilizing agonists appear to activate the Na+/H+ exchanger by at least two different mechanisms: (1) the alpha1-adrenoreceptor-mediated activation that is dependent on extracellular Ca2+ and PKC; and (2) vasoactive-peptide-induced alkalinization that is independent of extracellular Ca2+ and PKC. The alpha1-adrenoreceptor-mediated, PKC-sensitive, activation of the Na+/H+ exchanger seems to be responsible for the distinct ability of these receptors to elicit the sustained stimulation of hepatic functions.