Fig. 2. (A) The relationship between secretion flow rate (x axis) and Cl^- secretion rate (y axis) in perfused rectal gland preparations stimulated with 5x10^-6 mol l^-1 forskolin. y=0.52x-0.01 (r=0.99, N=127, P<0.0001). The slope (0.52) of the regression line indicates that the average concentration of Cl^- in the secretion was 520 mmol l^-1. Simultaneous measurements obtained at high perfusion pressure (20 mmHg), reduced perfusion pressure (12 mmHg), and after all experimental treatments at reduced perfusion pressure (12 mmHg), are plotted: N=127 data points from 71 preparations. (B). Open triangles: the relationship between Cl^- secretion rate (x) and oxygen consumption rate (; y) in all perfused rectal gland preparations. y=0.022x+0.11 (r=0.73, N=139, P<0.0001). Simultaneous measurements obtained at high perfusion pressure (20 mmHg) in the absence of stimulation (no Cl^- secretion), and in the presence of stimulation with 5.5x10^-6 mol l^-1 forskolin at high perfusion pressure (20 mmHg), at reduced perfusion pressure (12 mmHg), and after all experimental treatments at reduced perfusion pressure (12 mmHg), are plotted: N=139 data points from 71 preparations. Closed circles: the relationship between Cl^- secretion rate (x) and carbon dioxide excretion rate (; y) in perfused rectal gland preparations. y=0.036x+0.22 (r=0.48, N=117, P<0.0001). The same set of simultaneous measurements as used for are plotted, with the exception of points obtained at high perfusate HCO₃^- concentration (where it was impossible to resolve small arterial-venous differences in total CO₂ concentrations) and after treatment with acetazolamide (which transiently inhibited CO₂ excretion). N=117 data points from 68 preparations.