Progesterone induces a biphasic Ca2+ influx and consequent acrosome reaction in human spermatozoa. We have used two procedures to vary the stimulus (dosage and prior receptor desensitization) to investigate the encoding of stimulus strength by intracellular [Ca2+] ([Ca2+]i). Acrosome reaction and amplitude (but not kinetics) of the transient [Ca2+]i response (population measurement) showed sigmoidal dose sensitivity over the range 0.3 nM–3 μM, saturating at ≈300 nM (ED50≈30 nM). The amplitude of the sustained response saturated at 3 μM. Single-cell imaging showed that the amplitudes of both transient and sustained [Ca2+]i responses were highly dose-dependent, but that their frequency of occurrence and kinetics were largely dose-independent. Fluorimetric measurements confirmed that progesterone-induced [Ca2+]i influx was subject to desensitization, with second and subsequent applications of 3 μM progesterone being ineffective. However, sequential additions of 3 nM, 30 nM and 3 μM progesterone generated transient [Ca2+]i responses at each concentration, the amplitude and duration of the response to 3 μM progesterone being reduced compared with non-pretreated cells. Single-cell imaging revealed that pretreatment had no effect on the proportion of responsive cells, but single-cell responses, similarly to population responses, were smaller and markedly reduced in duration, consistent with an effect of desensitization on a late component of the [Ca2+]i transient. We conclude that the strength of the progesterone stimulus, when varied by dosage or by desensitization, is encoded by an analogue [Ca2+]i signal. Dose dependency of the acrosome reaction is therefore determined not by the number of progesterone-responsive cells but by variation in the probability of exocytosis in a ‘constant’ responsive population.

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