Molecular oxygen: friend and foe. The role of the oxygen free radical system in the calcium paradox, the oxygen paradox and ischemia/reperfusion injury

Norepinephrine (NE) and glutamate (Glu) initiate intracellular calcium ([Ca2+]i) transients, oscillations, and intracellular [Ca2+]i waves in cultured astrocytes. To further elucidate the significance of NE- and Glu-evoked astrocytic [Ca2+]i signaling to neuron-astrocyte communication in the mature CNS, [Ca2+]i of astrocyte networks within hippocampal slices (P21–42) was measured during bath application of NE and Glu receptor agonists. Astrocytes in stratum radiatum were identified by highly negative membrane potentials (75 +/- 3 mV), absence of action potentials, and dye coupling following intracellular injection of the [Ca2+]-sensitive dye calcium orange. NE (2–100 microM) evoked [Ca2+]i increases (7 of 8 slices, 24 of 24 cells in responding slices) characterized by an initial rise, 20–50 sec to peak, followed by a slower return to baseline (over approximately 8 min). The alpha 1- agonist phenylephrine (PE) (10–100 microM) evoked complex [Ca2+]i signals (22 of 26 slices, 90 of 90 cells in responding slices) composed of both a prolonged component (5.1 +/- 1.8 min), synchronized in neighboring cells, and multiple, mainly asynchronous [Ca2+]i spikes (25.0 +/- 11.6 sec). PE responses were completely blocked by the alpha 1-antagonist prazosin (200 nM, n = 4 slices), but not by the alpha 2- antagonist yohimbine (n = 3 slices). The alpha 2-agonist clonidine (10- 100 microM) did not increase [Ca2+]i (n = 4 slices). alpha 1-mediated [Ca2+]i transients were observed after removal of extracellular [Ca2+]o (n = 8 of 9 slices), indicating PE-induced Ca2+ release from intracellular stores. Adrenergic responses were mediated by alpha 1- receptors localized to astrocytes because PE and NE increased [Ca2+]i of acutely isolated hippocampal astrocytes. Glu (0.75–2.0 mM) did not increase astrocytic [Ca2+]i in slices (0 of 7), even in the presence of the Glu uptake inhibitor L-trans-pyrrollidine-2,4-dicarboxylic acid (PDC) (0 of 5 slices), or in acutely isolated astrocytes (0 of 7 cells). The metabotropic agonist t-ACPD (30 or 50 microM) did not increase astrocytic [Ca2+]i in hippocampal slices (0 of 5), while kainate (200 microM or 1 mM) induced brief (1–2 min) [Ca2+]i increases only rarely (2 of 8 applications in 6 slices). These results support a primary role of NE release and alpha 1-adrenoceptor stimulation in neuron-astrocyte communication in the mature CNS.

https://www.jneurosci.org/content/jneuro/15/8/5535.full.pdf

Adrenergic calcium signaling in astrocyte networks within the hippocampal slice

The three major calcium channel blocking agents, diltiazem, nifedipine and verapamil, inhibit calcium entry into excitable cells. Despite this apparent common action at the cell membrane, these drugs produce quantitative and frequently qualitative differences in cardiovascular variables (for example, heart rate, atrioventricular [A-V] conduction and myocardial inotropic state) when evaluated at equieffective vasodilator doses. All three drugs increase coronary blood flow in a dose-dependent fashion (nifedipine greater than diltiazem = verapamil), and produce a negative inotropic effect in vitro in isolated atria and ventricles, also in a dose-dependent manner (verapamil greater than nifedipine greater than diltiazem). However, in conscious dogs nifedipine increases, verapamil decreases and diltiazem has little effect on the inotropic state. A-V conduction is slowed by diltiazem and verapamil but not by nifedipine in anesthetized dogs and in conscious dogs as judged from the P-R interval in the electrocardiogram. Heart rate is slowed in pentobarbital-anesthetized animals but is accelerated in conscious dogs (nifedipine greater than verapamil greater than diltiazem). Nifedipine also appears to interfere significantly with the arterial baroreceptor reflex by an apparent vagolytic action that is less evident with diltiazem and verapamil. Diltiazem, and possibly verapamil and nifedipine as well, appears to retard myocardial damage that accompanies ischemia. The mechanisms and sites of action of these drugs are presumed to be at the cell membrane; however, intracellular sites may also be involved.

Differential cardiovascular effects of calcium channel blocking agents: Potential mechanisms*

The enantiomers of a racemic drug generally differ in their pharmacokinetic and/or pharmacodynamic properties. Because bupivacaine is a mixture of two optical isomers known to exert different toxic properties on isolated nerve preparations, we decided to use an isolated rabbit heart model with const

Myocardial Uptake of Bupivacaine: II. Pharmacokinetics and Pharmacodynamics of Bupivacaine Enantiomers in the Isolated Perfused Rabbit Heart

Diffusion of drugs in native and purified gastrointestinal mucus

Accumulation of drugs by resting or beating cardiac tissue.

An isolierten Herzvorhofpraparaten des Meerschweinchens wurde sowohl das Einsetzen und das Abklingen der Wirkung des Parasympatholyticums Dexetimid wie auch die Bindung und Freigabe dieser Substanz durch das Gewebe bei verschiedenen Schlagfrequenzen untersucht. Der pA2-Wert als Mas fur die Wirkstarke eines Antagonisten erwies sich als von der Schlagfrequenz unabhangig. Bei rhythmisch kontrahierenden Muskeln erfolgte die Bindung rascher und in einem groseren Ausmase als bei ruhenden Vorhofpraparaten. Bei hoheren Erregungsfrequenzen bildete sich der pharmakologische Effekt rascher aus. Die vermehrte mechanische Aktivitat des Gewebes wird als Ursache des beschleunigten und vermehrten Stofftransportes diskutiert.



The Influence of the Beat Frequency on the Time Course of Drug Binding and Pharmacological Action of Dexetimide in Atrial Muscle.



In isolated guinea pig auricles, the onset and the decline of the antimuscarinic action of dexetimide as well as binding and release of the compound by the tissue were studied as functions of the beat frequency. The pA2 value proved to be independent of the beat frequency. In comparison to quiescent muscles, the rate of drug binding and the extent of drug accumulation were found to be increased in beating preparations. At higher driving frequencies the development of the pharmacological effect was accelerated. The facilitated and enhanced drug transport is considered to be the consequence of the increased mechanical activity of the tissue.

Pieter B. M. W. M. Timmermans

Papers

Accumulation of drugs by resting or beating cardiac tissue.

Der Zeitverlauf der Bindung und Wirkung von Dexetimid an Herzvorhofgewebe unter dem Einfluß der Schlagfrequenz