Calcium and protein kinase C play an important role in Campylobacter jejuni-induced changes in Na⁺ and Cl^- transport in rat ileum in vitro

Abstract

The pathophysiological mechanism of Campylobacter jejuni (enterotoxigenic) induced secretory diarrhoea remains least understood. To investigate the mechanism(s) involved, the unidirectional fluxes of Na⁺ and Cl^- were measured across the C. jejuni live culture infected and control (non infected) rat ileum (unstriped), in vitro by Ussing technique under short circuit conditions, in the presence or absence of: Ca²⁺ ionophore A23187 (5 μM), 1-verapamil (100 μM), calmodulin (CaM) antagonist W-7 (100 μM), dantrolene (25 μM), protein kinase C (PKC) activator PMA (100 ng/ml) and H-7 (60 μM), selective inhibitor of PKC. There was net absorption of Na⁺ and enhanced Cl^- secretion in infected animals while in control animals there was net absorption of Na⁺ and marginal secretion Cl^-. Ca²⁺ ionophore A23187 mimicked the effects of C. jejuni infection whereas 1-verapamil had significant antisecretory effect on Na⁺ and Cl^- secretion in infected animals. In vitro measurement of unidirectional ⁴⁵Ca fluxes in Ussing chamber experiments revealed net absorption of Ca²⁺ in infected rat ileum as compared to net secretion of Ca²⁺ in control rat ileum. These observations clearly indicate that there is increased stimulation of Ca²⁺ uptake from extracellular milieu to the enterocytes during C. jejuni-induced diarrhoea. The intracellular calcium levels (Ca²⁺]_i (as measured by fluorescent probe Fura-2AM) were found to be raised significantly (P < 0.001) in enterocytes isolated from C. jejuni infected ileum as compared to the enterocytes from control ileum. The observed increase in [Ca²⁺]_i in enterocytes isolated from C. jejuni live culture supernatant treated rat ileum further shows the involvement of enterotoxin in diarrhoeal process. Dantrolene decreased significantly C. jejuni-induced net Na⁺ and Cl^- secretion but it could not reverse it to absorption suggesting the partial involvement of Ca²⁺ mobilised from intracellular stores in mediating secretion. W-7 failed to inhibit the C. jejuni-induced net Na⁺ and Cl^- secretion. In addition the CaM activity estimated in intestinal microvillar core remained same in both the control and C. jejuni infected animals. This indicates that C. jejuni-induced diarrhoea is not mediated through the activation of Ca^2+-CaM complex pathway of the Ca²⁺ messenger system. The PKC activator PMA, induced net secretion of Na⁺ and Cl^- in the control animals but it could not enhance further the C. jejuni-induced Na⁺ and Cl^- secretion, suggesting that there is overlapping effect of PMA and C. jejuni live culture infection. H-7 significantly inhibited (P < 0.001) net Na⁺ and Cl^- secretion in the infected animals but had no effect on basal rat ileal Na⁺ and Cl^- transport in control animals. This again suggest the activation of PKC signal in C. jejuni-induced ileal Na⁺ and Cl^- secretion. Although intracellular cAMP levels were found to be significantly raised in the enterocytes isolated from C. jejuni live culture infected and C. jejuni culture supernatant treated rat ileum, the mechanism of interaction between cAMP and Ca²⁺ messenger system in infection remains undefined. Taken together, the present results therefore, indicate that enterotoxigenic C. jejuni live culture-induced Na⁺ and Cl^- secretion is a calcium-dependent process where both increased Ca²⁺ entry from extracellular milieu and Ca²⁺ mobilization from intracellular stores result in increased enterocyte (Ca²⁺]_i. The activation of PKC further appears to be the important intracellular mediator pathway involved in Ca²⁺-dependent stimulation of C. jejuni-induced diarrhoea.