Intracellular Ca2+ transients in HT29 cells induced by hypotonic cell swelling

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Abstract
Cell swelling induced by hypotonic solution led to an osmolality-dependent increase in intracellular Ca2+ activity ([Ca2+]i) in HT29 cells. At moderate reductions in osmolality from 290 to 240 or 225 mosmol/l in most cases only a small monophasic increase of [Ca2+]i to a stable plateau of 10–20 nmol/l above resting [Ca2+]i was observed. Lower osmolalities resulted in a triphasic increase of [Ca2+]i to a peak value. In a first phase after the volume change, lasting 20–40 s, [Ca2+]i increased slowly by about 30 nmol/l. Thereafter [Ca2+]i increased more rapidly within 20–30 s to a peak value. This peak was 189±45 nmol/l (190 mosmol/l, n=9) and 243±41 nmol/l (160 mosmol/l, n=20) above resting [Ca2+]i. The peak was then followed by a decline of [Ca2+]i over the next 2–3 min to a stable plateau value of 28±6 (n=6) and 32±11 nmol/l (n=11) above resting [Ca2+]i at 190 and 160 mosmol/l, respectively. The plateau lasted as long as the hypotonic solution was present. Under Ca2+-free bath conditions the peak value for the cell-swelling-induced [Ca2+]i transient was reached significantly later (60–100 s, compared to 40–60 s under control conditions). The peak values under Ca2+-free conditions were not significantly lower. This indicates that the [Ca2+]i peak was mostly of intracellular origin. No [Ca2+]i plateau phase was observed under Ca2+-free bath conditions. With the use of the fura-2-Mn 2+ quenching technique an increased Ca2+ influx induced by hypotonic cell swelling was shown (160 mosmol/l; n=4). This influx started immediately after or simultaneously with the cell swelling and preceded the [Ca2+]i peak for more than 50 s.