CALCIUM INFLUX PATHWAYS IN RAT PANCREATIC DUCTS

A number of agonists increase intracellular Ca2+ activity, [Ca2+](i), in pancreatic ducts, but the influx/efflux pathways and intracellular Ca2+ stores in this epithelium are unknown. The aim of the present stu dy was to characterise the Ca2+ influx pathways, especially their pH s ensitivity, in native pancreatic ducts stimulated by ATP and carbachol , CCH. Under control conditions both agonists led to similar changes i n [Ca2+](i). However, these Ca2+ transients, consisting of peak and pl ateau phases, showed different sensitivities to various experimental m anoeuvres. In extracellular Ca2+-free solutions, the ATP-induced [Ca2](i) peak decreased by 25%, but the CCH-induced peak was unaffected; b oth plateaus were inhibited by 90%. Flufenamate inhibited the ATP-indu ced peak by 35%, but not the CCH-evoked peak; the plateaus were inhibi ted by 75-80%. La3+ inhibited the ATP-induced plateau fully, but that induced by CCH by 55%. In resting ducts, an increase in extracellular pH, pH(e), by means of HEPES and HCO3-/CO2 buffers, increased [Ca2+](i ); a decrease in pH(e) had the opposite effect, In stimulated ducts th e pH-evoked effects on Ca2+ influx were more pronounced and depended o n the agonist used. At pH(e) 6.5 both ATP- and CCH-evoked plateaus wer e inhibited by about 50%. At pH 8.0 the ATP-stimulated plateau was inh ibited by 27%, but that stimulated by CCH was increased by 72%. Taken together, we show that CCH stimulates Ca2+ release followed by Ca2+ in flux that is moderately sensitive to flufenamate, La3+, depolarisation , it is inhibited by low pH, but stimulated by high pH. ATP stimulates Ca2+ release and probably an early Ca2+ influx, which is more markedl y sensitive to flufenamate and La3+, and is both inhibited by low and high pH. Thus our study indicates that there are at least two separate Ca2+ influx pathways in pancreatic ducts cells.