Role of Guanine Nucleotide Regulatory Proteins and Inositol Phosphates in the Hormone Induced Mobilization of Hepatocyte Calcium
Many hormones and neurotransmitters exert their effects in the liver by increasing the concentration of free ionized Ca2+ in the cytoplasm.1,2,3 Examples of such agonists are norepinephrine and epinephrine acting via α1-adrenergic receptors, vasopressin acting on V1 receptors, ATP and ADP acting on P2 purinergic receptors and angiotensin II. Each agonist, after binding to its specific cell surface receptor, provokes the hydrolysis of PI 4,5-P2 by a phospholipase C activity to give rise to DAG and Ins 1,4,5-P3.4,5 The DAG functions to activate protein kinase C while Ins 1,4,5-P3 mobilizes Ca2+ from elements of the endoplasmic reticulum.4,5 Recent studies have implicated the involvement of a guanine nucleotide binding protein which couples the various hormone receptors to the PI 4,5-p2 specific phospholipase C6. The existence of a novel inositol phosphate ester, Ins 1,3,4,5-P4, has also been described.7 The purpose of this article is to present our data on these two aspects of hormone action in the liver.
KeywordsCholera Toxin Guanine Nucleotide Pertussis Toxin Inositol Phosphate GTPase Activity
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