Increased plasma binding and decreased blood cell binding of quinidine in blood from anuric rats.

The blood cell/plasma concentration ratio of quinidine, as influenced by the plasma protein binding, was studied in normal and anuric rats by applying incubation and equilibrium dialysis techniques on blood and plasma, respectively, from normal and anuric rats. The plasma protein binding of quinidine in anuria was increased at concentrations of unbound drug of less than 1.75 X 10(-4) M and decreased above this concentration. At an assumed "therapeutic" quinidine concentration (1 X 10(-5) M), the mean concentration ratio (total quinidine in blood cells)/(total quinidine in plasma) was 1.84 in normals and 0.46 in anuria, and the mean ratio (total quinidine in blood cells)/(unbound in plasma) was 4.45 and 1.81, respectively. As the latter ratios were concentration dependent and greater than could be accounted for by pH-dependent distribution, quinidine is presumably bound in/on the blood cells. Reduced distribution ratio in anuria, even when related to unbound quinidine in plasma, also indicates changed binding in blood cells, a finding confirmed by applying the data to modified Scatchard plot. this may have implication for the use of blood cell/plasma concentration ratio as screening procedure for the altered plasma binding of quinidine in patients.