Diurnal oscillations in the serum and salivary levels of quinidine.

The pharmacokinetics of quinidine in healthy subjects was studied through serum and salivary compartments by orally administering 250 mg of quinidine sulphate (CAS 50-54-4) either at 10:00 or 22:00 h. Quinidine concentrations in the biological samples were determined fluorimetrically. None of the pharmacokinetic parameters obtained from serum levels showed any significant time dependent changes. However, prolongation in tmax and lowering of Ka and t1/2 was noticed after nocturnal drug administration when the parameters were computed from salivary levels. Also, significant (p < 0.05) lowering of saliva/serum (SL/SR) ratios was observed after 22:00 h drug dosing. Temporal alterations in quinidine secretion into saliva could be the reason for the above changes. Alteration of salivary quinidine levels in turn could be caused by altered protein binding in serum and/or the time dependent changes in the permeability of biological membranes involved in the production of saliva.