Dependence of current simulations on BzATP and extracellular calcium concentrations. (A and B) Current simulations generated by the kinetic model, Eqs. 1–9, in the presence of 2 mM Ca²⁺ (A) and in the absence (B) of bath Ca²⁺ during 40-s and 2-min BzATP stimulation at increasing doses, as specified by the gray bars. Monophasic and biphasic responses are obtained at low and high concentrations of BzATP, respectively, in both cases, a result consistent with the experimental recordings (Fig. 1). The loss of allosteric inhibition exerted by extracellular Ca²⁺ in B led to an increase in the potency of BzATP, which manifested itself in requiring less agonist concentration (a reduction of ∼70% compared with A) to produce the same response.