cAMP and Ca²⁺ act co-operatively on the Cl⁻ conductance of HT₂₉ cells

Abstract

Previous studies in HT₂₉ cells have revealed that the Cl⁻ channels induced by cAMP or by increasing cytosolic Ca²⁺, e.g. by addition of ATP, and by hypotonic cell swelling share in common all examined properties, such as ion selectivity and blocker sensitivity. In addition, it was shown that conductances induced by either pathway were not additive. Therefore all three pathways apparently act on the same type of small conductance Cl⁻ channel. In CFPAC-1 cells the general properties of the Cl⁻ conductance were identical. However, the cAMP response was absent. In both cell types the Ca²⁺-mediated conductance response was transient. Here we examine the kinetics of the conductance increases induced by neurotensin (NT, 10⁻⁸ mol/l) or ATP (10⁻⁵ mol/l) in HT₂₉ and CFPAC-1 cells using the slow (nystatin) or fast whole cell patch clamp technique, and we ask whether cAMP influences these kinetics. In the continuous presence of NT the conductance response in both cell types was very transient. It collapsed with a time constant (τ) of 39 (30–56 s) in HT₂₉ and of 33 (27–41 s) in CFPAC-1 cells. The ATP response was also transient with a τ of 49 (42–57 s) in HT₂₉ cells and 102 (77–152 s) in CFPAC-1 cells. Pre-treatment by membrane permeable cAMP (10⁻³ mol/l) enhanced the baseline conductance in HT₂₉ but not in CFPAC-1 cells. Furthermore, the ATP- and NT-induced conductance increases became significantly less transient in HT₂₉ but not in CFPAC-1 cells. In the former cells τ was enhanced significantly to 207 (154–316 s) after ATP and to 1.533 (1004-∞ s) after NT. In CFPAC-1 cells the transient nature of the conductance response persisted. These data indicate that cAMP and Ca²⁺ co-operate in HT₂₉- but not in CFPAC-1-cells. In the former cells the transient conductance response is converted into a more stable response by cAMP. In CFPAC-1 cells the cAMP-mechanism is not functioning. Therefore, all Ca²⁺-mediated conductance responses are only very transient.