Figure 8. Reduced Ca²⁺ sensitivity and cooperativity of neuromuscular synaptic transmission in Syb1^lew/lew mice.

A, sample EPP traces recorded at various external [Ca²⁺] (0.5, 1.0, 2.0 and 4.0 mm). Each panel represents 10 superimposed EPP traces, with peaks indicated by open circles on the left side. In both control (A) and Syb1^lew/lew mice (B), EPP amplitudes decrease and variability increases as external [Ca²⁺] is reduced. C and D, dose–response curve of average EPP amplitude (C) and quantal content (D), as a function of external [Ca²⁺], from recordings of P14 diaphragm muscles from Syb1^lew/lew and control mice. The curve is fitted by a sigmoidal dose–response equation: , where Y is the value of the EPP amplitude or the quantal content at the indicated [Ca²⁺], Y_max and Y_min are the maximum and minimum EPP amplitude or quantal content, respectively, EC₅₀ is the [Ca²⁺] when Y is 50% of Y_max, and k is the Hill coefficient (Hill slope), which reflects the steepness of the curve. The sigmoidal dose–response curve of EPP amplitude and quantal content as a function of external [Ca²⁺] showed that the EC₅₀ was markedly increased and the Hill slope was markedly reduced in Syb1^lew/lew NMJs (n = 29) compared with the control NMJs (n = 23).