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

Increases of cytosolic Ca2+, as occur with agonists such as ATP, neurotensin (NT), hypotonic cell swelling and ionomycin, enhance the membrane conductance (G(M)) and hence the input conductance (G(I)) of HT29 cells. In the present study we have examined whether these increases in G(M) are paralleled by exocytosis. To this end the membrane capacitance (C(M)) of HT29 cells was measured by patch clamp techniques. Two methods to monitor C(M) were used: a direct method (DM) and a phase tracking 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 respectively. 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 mosmol/l, n = 18) had a comparable effect: G(M) and C(M) were increased from 4.9 +/- 1 nS and 30 +/- 4.1 pF to 46 +/- 10 nS and 37 +/- 4.9 pF respectively. 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 in the case of ATP, and from 20.6 +/- 2.3 to 24.3 +/- 2.6 pF in the case of NT). The present data indicate that transient (ATP, NT) or more stable (hypotonic cell swelling, ionomycin) increases in [Ca2+]i produce corresponding increments in G(M) and C(M). The relative changes in both parameters correlate with each other. These findings are compatible with the view that exocytosis is related to the Ca2+-mediated control of Cl- conductance.