Epac1 mediates protein kinase A-independent mechanism of forskolinactivated intestinal chloride secretion

Kazi Mirajul Hoque, Owen M. Woodward, Damian B. Van Rossum, Nicholas C. Zachos, Linxi Chen, George P.H. Leung, William B. Guggino, Sandra E. Guggino, Chung Ming Tse

Research output: Contribution to journalArticlepeer-review

Abstract

Intestinal Cl- secretion is stimulated by cyclic AMP (cAMP) and intracellular calcium ([Ca2+]i). Recent studies show that protein kinase A (PKA) and the exchange protein directly activated by cAMP (Epac) are downstream targets of cAMP. Therefore, we tested whether both PKA and Epac are involved in forskolin (FSK)/cAMP-stimulated Cl- secretion. Human intestinal T84 cells and mouse small intestine were used for short circuit current (Isc) measurement in response to agonist-stimulated Cl - secretion. FSK-stimulated Cl- secretion was completely inhibited by the additive effects of the PKA inhibitor, H89 (1 μM), and the [Ca2+]i chelator, 1,2-bis-(o-aminophenoxy)-ethane-N,N, N′,N′-tetraacetic acid, tetraacetoxymethyl ester (BAPTA-AM; 25 ìM). Both FSK and the Epac activator 8-pCPT-2′-O-Me-cAMP (50 μM) elevated [Ca2+]i, activated Ras-related protein 2, and induced Cl- secretion in intact or basolateral membrane-permeabilized T84 cells and mouse ileal sheets. The effects of 8-pCPT-2′-O-Me-cAMP were completely abolished by BAPTA-AM, but not by H89. In contrast, T84 cells with silenced Epac1 had a reduced Isc response to FSK, and this response was completely inhibited by H89, but not by the phospholipase C inhibitor U73122 or BAPTA-AM. The stimulatory effect of 8-pCPT-2′-O-Me-cAMP on Cl - secretion was not abolished by cystic fibrosis transmembrane conductance (CFTR) inhibitor 172 or glibenclamide, suggesting that CFTR channels are not involved. This was confirmed by lack of effect of 8-pCPT-2′-O-Me- cAMP on whole cell patch clamp recordings of CFTR currents in Chinese hamster ovary cells transiently expressing the human CFTR channel. Furthermore, biophysical characterization of the Epac1-dependent Cl- conductance of T84 cells mounted in Ussing chambers suggested that this conductance was hyperpolarization activated, inwardly rectifying, and displayed a Cl ->Br->I- permeability sequence. These results led us to conclude that the Epac-Rap-PLC-[Ca2+]i signaling pathway is involved in cAMP-stimulated Cl- secretion, which is carried by a novel, previously undescribed Cl- channel.

Original languageEnglish (US)
Pages (from-to)43-58
Number of pages16
JournalJournal of General Physiology
Volume135
Issue number1
DOIs
StatePublished - Jan 2010

ASJC Scopus subject areas

  • Physiology

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