Effects of tilisolol on ischemic myocardial metabolism in dogs

Effects of Propranolol and Verapamil on Changes in TQ and ST Segment Potentials During Graded Coronary Flow Reduction in a Porcine Myocardial Ischemia Model

Acute myocardial ischemia causes TQ depression and ST elevation. However, the effects of cardioprotective drugs such as β-blockers and Ca⁺⁺-antagonists on the extent of TQ depression, ST elevation, and myocardial ischemic injury are not fully understood.We created a carotid-coronary shunt in 30 pigs, and extracellular K⁺ ([K⁺]_e), TQ, and ST segments were recorded simultaneously with K⁺-selective plunge electrodes placed in the left anterior descending artery (LAD) distribution during graded LAD flow reduction before and after administration of propranolol or verapamil. Unipolar DC-coupled electrograms were recorded from the reference pole of the K⁺-selective plunge electrodes. The microvolt readings from the K⁺-selective electrodes were converted to [K⁺]_e and then to the changes in potassium equilibrium potential (ΔE_K). The shunted LAD flow was reduced in a stepwise fashion at 5-minute intervals.segment depression at the similar ΔE_K was not affected by propranolol or verapamil. However, ST segment elevation was reduced by propranolol but exacerbated by verapamil at the similar ΔE_K.TQ-ST changes recorded by AC coupled ECG are not a reliable index of ischemia and therefore cannot be used to evaluate the effects of drugs that might affect the electrophysiologic properties of ischemic myocardium.

Effects of tilisolol, a nonselective beta-adrenergic blocker, on the membrane currents of isolated guinea pig ventricular myocytes

The effects of tilisolol, a nonselective beta-adrenoceptor blocker, on transmembrane ionic currents were studied in single guinea pig ventricular myocytes by using the whole-cell voltage clamp technique. In the absence of beta-adrenergic stimulation, 10 microM tilisolol, a concentration higher than that used in the clinical therapeutic regimen, did not affect the L-type Ca2+ current (ICa), the inwardly rectifying K+ current (IK1), or the delayed rectifying K+ current (IK). In addition, it did not induce currents through the adenosine triphosphate (ATP)-sensitive K+ channels. However, under the nonselective beta-adrenergic stimulation with 1 microM isoproterenol, 1 microM tilisolol almost completely reversed the agonist-induced increase of IK. The increase of ICa by isoproterenol was blocked only by approximately 30% with tilisolol. We concluded that, at therapeutic concentrations (0.01-0.15 microM), tilisolol is a pure beta-adrenoceptor antagonist that has no direct effects on the transmembrane ionic currents of mammalian ventricular myocytes, such as ICa, IK1, or IK. Comparison of the dose-dependent effects of tilisolol on ICa and IK suggested that tilisolol may selectively inhibit catecholamine-induced increase of IK at the therapeutic concentrations. The virtually selective inhibition of IK, leaving ICa intact, may be favorable to prevent the catecholamine-induced arrhythmia without inhibiting contraction.

Role of the sympathetic nervous system in the ischemic and reperfused heart

Norepinephrine, that has been released from sympathetic nerve endings in response to myocardial ischemia, may have either a beneficial or a harmful effect on the ischemic heart. If the duration of ischemia is short, the release of norepinephrine may be favorable for the production of energy and for protection of the heart against ischemic damage. If the duration of ischemia is prolonged, there is a marked increase in number of both alpha 1 and beta-adrenoceptors located in the sarcolemmal membrane, as well as an excessive increase in release of norepinephrine. These events during the prolonged period of ischemia can produce an imbalance between oxygen supply and demand, which is harmful to the heart. The anti-ischemic effect of alpha 1- and beta-adrenoceptor antagonists is not attributed merely to improvement of oxygen balance, but reduction of phospholipase activity or stabilization of membrane may also be important as an underlying mechanism.