INCREASE IN CYTOSOLIC CA2- CONDUCTANCE IN HT29 CELLS( REGULATES EXOCYTOSIS AND CL)

Increases of cytosolic Ca2+, as occur with agonists such as ATP, neuro tensin (NT), hypotonic cell swelling and ionomycin, enhance the membra ne conductance (G(M)) and hence the input conductance (G(I)) of HT29 c ells. In the present study we have examined whether these increases in G(M) are paralleled by exocytosis. To this end the membrane capacitan ce (C(M)) of HT29 cells was measured by patch clamp techniques. Two me thods to monitor C(M) were used: a direct method (DM) and a phase trac king method (PTM). With the DM the following results were obtained. NT (10(-8) mol/l, n = 9) increased G(M) and C(M) significantly from 2.4 +/- 0.3 nS and 23.5 +/- 3 pF to 32 +/- 8 nS and 27.3 +/- 3.1 pF respec tively. ATP (10(-4) mol/l, n = 29) had a very similar effect. G(M) and C(M) were increased from 5.7 +/- 1 nS and 36 +/- 4.4 pF to 111 +/- 21 nS and 44 +/- 5.4 pF respectively. Hypotonic cell swelling (160 mosmo l/l, n = 18) had a comparable effect: G(M) and C(M) were increased fro m 4.9 +/- 1 nS and 30 +/- 4.1 pF to 46 +/- 10 nS and 37 +/- 4.9 pF res pectively. Ionomycin (10(-7) mol/l, n = 4) gave similar results. With the PTM it was possible to monitor the rapid changes in G(M) and C(M), as they were induced by ATP (n = 42) and NT (n = 29), with high time resolution. The transient and instantaneous (< 1 s) increases in G(I) (from 2.1 +/- 0.4 to 21.7 +/- 1.7 nS in the case of ATP, and from 2.3 +/- 0.4 to 26.6 +/- 3.1 nS in the case of NT) were closely paralleled by transient increases in C(M) (from 17.6 +/- 1.4 to 21.1 +/- 1.7 pF i n the case of ATP, and from 20.6 +/- 2.3 to 24.3 +/- 2.6 pF in the cas e of NT). The present data indicate that transient (ATP, NT) or more s table (hypotonic cell swelling, ionomycin) increases in Ca2+!i produc e corresponding increments in G(M) and C(M). The relative changes in b oth parameters correlate with each other. These findings are compatibl e with the view that exocytosis is related to the Ca2+-mediated contro l of Cl- conductance.