Modulation of pancreatic acinar cell to cell coupling during ACh-evoked changes in cytosolic Ca2+

The temporal changes in cytosolic free Ca2+ ([Ca2+](i)), Ca2+-dependent mem brane currents (I-m), and gap junctional current (I-j) elicited by acetylch oline (ACh) were measured in rat pancreatic acinar cells using digital imag ing and dual perforated patch-clamp recording. ACh (50 nM-5 mu M) increased [Ca2+](i) and evoked I-m currents without altering I-j in 19 of 37 acinar cell pairs. Although [Ca2+], rose asynchronously in cells comprising a clus ter, the delay of the [Ca2+](i) responses decreased with increasing ACh con centrations, Perfusion of inositol 1,4,5-trisphosphate (IP3) into one cell of a cluster resulted in [Ca2+](i) responses in neighboring cells that were not necessarily in direct contact with the stimulated one. This suggests t hat extensive coupling between acinar cells provides a pathway for cell-to- cell diffusion of Ca2+-releasing signals. Strikingly, maximal (1-5 mu M) AC h concentrations reduced I-j by 69 +/- 15% (n = 9) in 25% of the cell pairs subjected to dual patch-clamping. This decrease occurred shortly after the I, peak and was prevented by incubating acinar cells in a Ca2+ free medium , suggesting that uncoupling was subsequent to the initiation of the Ca2+-m obilizing responses. Depletion of Ca2+-sequestering stores by thapsigargin resulted in a reduction of intercellular communication similar to that obse rved with ACh. In addition, ACh-induced uncoupling was prevented by blockin g nitric oxide production with L-nitro-arginine and restored by exposing ac inar cells to dibutyryl cGMP. The results suggest that ACh-induced uncoupli ng and capacitative Ca2+ entry are regulated concurrently. Closure of gap j unction channels may occur to functionally isolate nearby cells differing i n their intrinsic sensitivity to ACh and thereby to allow for sustained act ivity of groups of secreting cells.