Distinct subpopulations in HaCaT cells as revealed by the characteristics of intracellular calcium release induced by phosphoinositide-coupled agonists

Abstract Intracellular calcium release induced by transient applications of phosphoinositide agonists was measured using adherent single HaCaT keratinocytes loaded with the acetoxymethyl derivative of fura-2. Application of ATP, bradykinin and formyl-Met-Leu-Phe (fMLP) resulted in a transient increase in intracellular calcium concentration ([Ca²⁺]_i) with an average half-width of 40 ± 21 s and a decay time constant of 15 ± 10 s (mean ± SD, n = 108), irrespective of the agonist applied. The cells could be classified into two groups: in 53% of the cells repeated stimulation brought about a progressively smaller change in [Ca²⁺]_i (type 1 cells), whereas in the remaining cells the amplitude of the calcium transients was essentially unchanged (type 2 cells). Furthermore, calcium transients in type 1 cells had broader half-widths and slower decays. No difference was found between the agonists in respect of the characteristics of the evoked calcium transient within each subpopulation. However, bradykinin and fMLP desensitized some cells. These results indicate that the activation of the inositol trisphospate transduction pathway by different agonists induces a characteristic elevation of [Ca²⁺]_i within a given cell. Our results demonstrate that cultured HaCaT keratinocytes are heterogeneous in respect of the calcium transients evoked by the activators of this second messenger system.