Figure 9.34 The influence of vehicle pH on the aqueous humour concentration of pilocarpine and glycerol. •, 1002 mol dm03 pilocarpine solutions; A, 0.14 mol dm03 glycerol solutions; all samples obtained at 20 min post-instillation and represent results from a minimum of six eyes.

Reproduced from reference 16.

of drug in the bathing fluid; loss of drug solution through the punctae and nasolacrimal ducts does not affect concentration, as the whole fluid drains away. Solution concentration is reduced only by diffusion of drug across the cornea or conjunctiva or by tear inflow.

The pH5 solution induced more tear flow than the pH 8 solution; thus the concentration gradient is reduced and transport of both ionised and nonionic drugs is less at pH 5. At pH 11, as will be seen from Fig. 9.34, absorption of pilocarpine is reduced because of the

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