Clinical pharmacologic observations on timolol. II. Antihypertensive effect and kinetic disposition on twice-daily dosing in patients with mild or moderate hypertension

R-wave amplitude changes measured by electrocardiography during early transmural ischemia

BACKGROUND

Changes in the amplitude of the R wave (RWA) on the electrocardiogram (ECG) have been described during acute myocardial ischemia and infarction. However, this has not been well studied in a controlled setting. We hypothesized that significant increase in RWA occurs during early transmural myocardial ischemia.

METHODS

We prospectively evaluated changes in RWA in 50 patients during brief episodes of transmural ischemia induced by first balloon occlusion (mean, 38 seconds at 6-10 atmospheric pressures) during elective percutaneous coronary intervention. We recorded 12-lead ECGs at 20-second intervals before and during balloon inflation in 16 right coronary arteries, 14 left circumflex arteries, and 20 left anterior descending arteries. R wave amplitude was digitally measured in each of the 12 leads in every ECG using the ECG interval editor (General Electric HC, Menomonee Falls, WI). Intracoronary (IC) ECGs were also recorded in 4 patients. The mean of the RWA in each lead before balloon inflation was compared to the mean RWA during balloon inflation.

RESULTS

R wave amplitude significantly increased during balloon inflation from baseline in limb leads I, II, aVL, and all the precordial leads with the exception of lead V(1). The RWA increase did not reach statistical significance in leads III, aVF, and V(1). Mean RWA increase was consistent in all leads except aVR during the brief episodes of ischemia during initial balloon inflation because of the inverse polarity of this lead. The increase in RWA was seen in most patients (mean, 75%) in whom transmural ischemia was induced by first balloon inflation. Besides, the RWA showed an increase from baseline in 3 patients who had IC-lead recordings.

CONCLUSION

R wave amplitude increases significantly in precordial leads (V(2)-V(6)) and limb leads (I, II, aVL) of the surface ECG during brief episodes of transmural ischemia. The increase in RWA might be consistent with the expansion of the left ventricular cavity during ischemia and/or alterations in conduction that are intrinsic to the myocardium.

Assay of timolol in human plasma using gas chromatography with electron-capture detection

A simple and sensitive gas chromatographic method has been developed for the determination of timolol in plasma using electron-capture detection and propranolol as internal standard. Timolol was extracted using butyl chloride and derivatized using trifluoroacetic anhydride in butyl acetate. The lower detection limit for the assay was found to be 1 ng/ml from 1 ml of plasma. Extracted standards gave within-day precision of 12.55, 9.68 and 3.78% for 1, 20 and 100 ng/ml plasma samples, respectively. A recovery of at least 80% of timolol was found using the extraction method described. The assay was used in a randomized cross-over bioequivalence trial using an oral administration of 20 mg of timolol. Pharmacokinetic parameters compare favourably with other literature values.

A dose ranging study of atenolol in hypertension: fall in blood pressure and plasma renin activity, beta-blockade and steady-state pharmacokinetics

The relationship between the oral dosage and plasma concentration of the long-acting cardioselective beta-adrenoceptor blocker atenolol and the antihypertensive response to the the degree of beta-adrenoceptor blockade and change in plasma renin activity (PRA) was evaluated in patients with mild-to-moderate essential hypertension in a double-blind, randomized, between-patient, dose-ranging (25, 50 or 100 mg once daily for 4 weeks) study. The optimum, or minimum, daily dose of atenolol to treat patients with mild-to-moderate hypertension was not clearly identified in this study. A between-treatment comparison did not demonstrate that all blood pressure falls were always less in the 25 mg group than in the other two groups. Calculation of beta-error or the power for the negative results between doses suggested that a large sample size is required to draw a conclusion that no dose-antihypertensive relationship of atenolol exists in the treatment of mild-to-moderate hypertension. A relatively flat plasma concentration-antihypertensive response relationship was observed. Steady-state plasma concentrations of atenolol were dose-related and renal drug clearance was well correlated with individual creatinine clearance. beta-adrenoceptor blockade was better correlated with plasma atenolol concentration. Correlations which were less strong were between plasma drug concentration and change in various blood pressures and between blood pressure falls and beta-adrenoceptor blockade. There was no relationship between the fall in blood pressure and change in PRA. Atenolol appeared to suppress PRA in an all-or-none fashion.

Relationships among timolol doses, plasma concentrations and beta-adrenoceptor blocking activity

1 We investigated the relationships among dose, plasma concentration and beta-adrenoceptor blockade after single and repeated doses (0.5-20 mg) of timolol and placebo in six normal men. 2 Maximal suppression of exercise-induced tachycardia (bicycle ergometry) was dose-dependent and greater at 2 than at 6 h after dosing; activity up to 12 h was evident on the last dosing day. 3 Attenuation of exercise-induced tachycardia was strongly correlated with the timolol dosage over the 0.5 to 5-10 mg range. 4 A plasma timolol concentration of 27 ng/ml was associated with maximal suppression of exercise-induced tachycardia. 5 Suppression of exercise-induced tachycardia and areas under the plasma concentration-time curves did not differ significantly on the first and last dosing days.

Pharmacokinetics, mechanisms of action, indications, and adverse effects of timolol maleate, a nonselective beta-adrenoreceptor blocking agent

Timolol, a nonselective beta-adrenoreceptor blocking agent without intrinsic sympathomimetic or membrane stabilizing activity, has been shown effective in the treatment of angina and hypertension. It is particularly useful in patients with stable angina pectoris and patients with mild to moderate hypertension. In both of these conditions, timolol appears to be comparable to propranolol. A recent study has suggested that timolol reduces mortality and reinfarction rate in patients who have recently had a myocardial infarction. When given topically timolol reduces intraocular pressure in patients with open-angle glaucoma; the drug may be used as the primary agent or as an adjunct to standard therapy. Careful selection of patients will reduce the frequency of adverse effects due to beta-receptor inhibition. Thus, timolol should not be used in patients who are predisposed to asthmatic bronchitis or cardiac failure, and it should be used with caution in patients with peripheral vascular disease or diabetes mellitus.

Inhibition of angiotensin II potentiation of sympathetic nerve activity by beta-adrenergic antagonists

Since beta-adrenergic blockers are effective in the therapy of hypertension by a mechanism related to the degree of activation of the renin-angiotensin system, the effect of eight beta blockers was examined on angiotensin II potentiation of nerve stimulation (NS) in isolated perfused rat mesenteric vessels. The vasoconstrictor response to periarterial NS was obtained by monitoring changes in perfusion pressure while a beta blocker or a beta blocker and angiotensin II (3 ng/ml) were added to the perfusate. Although each beta blocker tended to decrease responses to NS, in the concentrations used, only metoprolol significantly inhibited responses to NS. Angiotensin II, when infused alone, potentiated the responses to NS by 63% (p less than 0.01). These enhanced responses following angiotensin II were inhibited in a dose-related manner (10--300 ng/ml) by beta 1, beta 2, and mixed beta blockers. At the 100 ng/ml concentration, DL-propranolol, timolol, metoprolol, practolol, butoxamine, and H35/25 inhibited the angiotensin II potentiation of NS by 83%, 76%, 77%, 59%, 72%, and 41% respectively. The order of potency for this action was as follows: timolol = metoprolol = butoxamine greater than propranolol greater than practolol greater than H35/25. Administration of D- and L-propranolol also reduced the responses by 75%. The vasoconstrictor responses to injected norepinephrine (NE), in the presence and absence of angiotensin II, were not altered by DL-propranolol or timolol. In conclusion, beta-adrenergic blockers were found to interfere with the effect of angiotensin II on the sympathetic neuron, a property that could contribute to the antihypertensive action of these drugs.