Fig. 1. Quantal Ca²⁺ release in a single smooth muscle cell. (A) In a Ca²⁺-free solution, carbachol (CCh; 50 µM; Ac) increased [Ca²⁺]_c (Ab); in its continued presence, [Ca²⁺]_c returned to near-basal levels. A tenfold higher concentration of CCh (500 µM; Ac) evoked a further release of Ca²⁺. [Ca²⁺]_c changes are represented by the colour changes in the frames i-v (Aa; blue low and yellow/red high [Ca²⁺]_c) and by the fluorescence transients (F/F₀; Ab). The images in the frames in Aa were taken before 50 µM CCh (i), during 50 µM CCh (ii,iii) and during 500 µM CCh (iv,v). The time-points at which images were obtained are indicated by their respective numerals (i-v) above the Ca²⁺ transients (Ab); these numerals correspond to those in Aa. The scale bar in the bright-field image of the cell (Aa) applies to all frames. (B) Essentially identical results were obtained in the presence of external Ca²⁺ except that [Ca²⁺]_c oscillations occurred with 500 µM CCh. These presumably arose, at least in part, from Ca²⁺ entry. Figures in Ac and Bb refer to concentrations of CCh applied to the cell by pressure ejection. In these experiments, fluorescence was measured in a region that encompassed the entire cell.