Increase in cytosolic Ca²⁺ regulates exocytosis and Cl⁻ conductance in HT₂₉ cells

Abstract

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.