Pattern of Ca 2+ increase determines the type of secretory mechanism activated in dog pancreatic duct epithelial cells

Intracellular calcium concentration ([Ca 2+ ] i ) is a key factor controlling secretion from various cell types. We investigated how different patterns of [Ca 2+ ] i signals evoke salt secretion via ion transport mechanisms and mucin secretion via exocytosis in dog pancreatic duct epithelial cells (PDEC). Activation of epithelial P2Y 2 receptors by UTP generated two patterns of [Ca 2+ ] i change: 2–10 μ m UTP induced [Ca 2+ ] i oscillations, whereas 100 μ m UTP induced a sustained [Ca 2+ ] i increase, both in the micromolar range. As monitored by carbon‐fibre amperometry, the sustained [Ca 2+ ] i increase stimulated a larger increase in exocytosis than [Ca 2+ ] i oscillations, despite their similar amplitude. In contrast, patch‐clamp recordings revealed that [Ca 2+ ] i oscillations synchronously activated a K + current as efficiently as the sustained [Ca 2+ ] i increase. This K + current was mediated by intermediate‐conductance Ca 2+ ‐activated K + channels (32 pS at −100 mV) which were sensitive to charybdotoxin and resistant to TEA. Activation of these Ca 2+ ‐dependent K + channels hyperpolarized the plasma membrane from a resting potential of −40 mV to −90 mV, as monitored in perforated whole‐cell configuration, in turn enhancing Na + ‐independent, Cl − ‐dependent and DIDS‐sensitive HCO 3 − secretion, as monitored through changes in intracellular pH. PDEC therefore encode concentrations of purinergic agonists as different patterns of [Ca 2+ ] i changes, which differentially stimulate K + channels, the Cl − –HCO 3 − exchanger, and exocytosis. Thus, in addition to amplitude, the temporal pattern of [Ca 2+ ] i increases is an important mechanism for transducing extracellular stimuli into different physiological effects.