Pattern of Ca²⁺ increase determines the type of secretory mechanism activated in dog pancreatic duct epithelial cells

Abstract

Intracellular calcium concentration ([Ca²⁺]_i) is a key factor controlling secretion from various cell types. We investigated how different patterns of [Ca²⁺]_i signals evoke salt secretion via ion transport mechanisms and mucin secretion via exocytosis in dog pancreatic duct epithelial cells (PDEC). Activation of epithelial P2Y₂ receptors by UTP generated two patterns of [Ca²⁺]_i change: 2–10 μm UTP induced [Ca²⁺]_i oscillations, whereas 100 μm UTP induced a sustained [Ca²⁺]_i increase, both in the micromolar range. As monitored by carbon-fibre amperometry, the sustained [Ca²⁺]_i increase stimulated a larger increase in exocytosis than [Ca²⁺]_i oscillations, despite their similar amplitude. In contrast, patch-clamp recordings revealed that [Ca²⁺]_i oscillations synchronously activated a K⁺ current as efficiently as the sustained [Ca²⁺]_i increase. This K⁺ current was mediated by intermediate-conductance Ca²⁺-activated K⁺ channels (32 pS at −100 mV) which were sensitive to charybdotoxin and resistant to TEA. Activation of these Ca²⁺-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 HCO₃⁻ secretion, as monitored through changes in intracellular pH. PDEC therefore encode concentrations of purinergic agonists as different patterns of [Ca²⁺]_i changes, which differentially stimulate K⁺ channels, the Cl⁻–HCO₃⁻ exchanger, and exocytosis. Thus, in addition to amplitude, the temporal pattern of [Ca²⁺]_i increases is an important mechanism for transducing extracellular stimuli into different physiological effects.