Role of intracellular calcium and protein kinases in the activation of hepatic Na super(+)/taurocholate cotransport by cyclic AMP

• Published in: The Journal of biological chemistry 1993-01, Vol.268 (24), p.17734-17741
• Main Authors: Gruene, S, Engelking, L R, Anwer, MS
• Format: Article
• Language: English
• ISSN: 0021-9258

Glucagon and dibutyryl cyclic AMP (Bt sub(2)cAMP) stimulate Na super(+)/taurocholate (TC) cotransport and increase the intracellular Ca super(2+) concentration ([Ca super(2+)] sub(i)) of hepatocytes. Whether the effect of cAMP is mediated via increases in [Ca super(2+)] sub(i), cAMP-dependent protein kinase (PKA), and/or protein kinase C (PKC) was investigated in this study. TC uptake and [Ca super(2+)] sub(i) were determined in isolated rat hepatocytes using [ super(14)C]TC and the fluorescent dye quin-2, respectively. Bt sub(2)cAMP, forskolin, and 8-bromo-cAMP stimulated Na super(+)-dependent, but not Na super(+)-independent TC uptake. Bt sub(2)cAMP increased the maximal rate of Na super(+)/TC cotransport without affecting the apparent K sub(m). Increases in TC uptake and [Ca super(2+)] sub(i) by Bt sub(2)cAMP were inhibited in hepatocytes preloaded with bis-(2-amino-5-methylphenoxy)-ethane-N,N,N',N'-tetracetic acid (MAPTA) or preincubated with 8-diethylaminooctyl 3,4,5-trimethoxybenzoate (TMB8). Calmodulin antagonists inhibited Bt sub(2)cAMP-induced increases in TC uptake, but not [Ca super(2+)] sub(i). Other Ca super(2+)-mobilizing agents (thapsigargin, vasopressin, phenylephrine, and ionomycin) increased [Ca super(2+)] sub(i) but failed to stimulate TC uptake, indicating that an increase in [Ca super(2+)] sub(i) alone is not a sufficient stimulus for TC uptake. However, increases in TC uptake by 1 and 10 mu M Bt sub(2)cAMP were further increased by thapsigargin, indicating a permissive role for Ca super(2+)/calmodulin. Bt super(2)cAMP-induced increases in TC uptake and [Ca super(2+)] sub(i) were inhibited by known inhibitors of PKA and by an activator of PKC, but they remained unaffected by a specific inhibitor of PKC. Unlike thapsigargin, vasopressin inhibited Bt super(2)cAMP-induced increases in TC uptake.