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  • Title: Beta-blocking effects of timolol at low plasma concentrations.
    Author: Kaila T, Huupponen R, Karhuvaara S, Havula P, Scheinin M, Iisalo E, Salminen L.
    Journal: Clin Pharmacol Ther; 1991 Jan; 49(1):53-8. PubMed ID: 1846331.
    Abstract:
    The concentration-effect relationship of 0.25 mg intravenous timolol with and without pretreatment with 100 mg quinidine was studied in six healthy young volunteers with a randomized, double-blind, crossover study design. Blockade of cardiac beta-adrenoceptors was assessed by determining the dose ratios (DR) of isoproterenol infusions required to increase heart rate by 25 beats/min before and after timolol infusion. The logarithm of timolol concentration in plasma was linearly related to the logarithm (DR-1) of isoproterenol infusion, with a mean Pearson correlation coefficient of 0.89 +/- 0.11 (+/- SD; n = 24) at timolol concentrations well below 1 ng/ml. The increases in cyclic adenosine monophosphate (cAMP) and norepinephrine plasma levels caused by isoproterenol infusions were attenuated after timolol. Quinidine administration increased timolol plasma levels and cardiac beta-blocking effects by 10% to 40%. It was concluded that timolol at concentrations below 1 ng/ml in plasma competitively antagonizes cardiac and noncardiac effects of isoproterenol infusions. Timolol effects are augmented after quinidine administration. The beta-blockade occurring at low plasma levels can explain side effects and actions of ocularly applied timolol.
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