ber den Mechanismus der positiv inotropen Wirkung von Theophyllin am Warmbl�terherzen: II. Wirkung von Theophyllin auf Aufnahme und Abgabe von45Ca

Synaptic potentials in sympathetic ganglia: are they mediated by cyclic nucleotides?

The hypothesis that cyclic nucleotides are intracellular second messengers mediating the generation of synaptic potentials was studied in the sympathetic ganglia of the bullfrog. Synaptic potentials and the effect of administering cyclic nucleotides and agents which affect cyclic nucleotide metabolism were recorded by the sucrose gap technique. The administration of adenosine 3',5'-monophosphate (cyclic AMP), guanosine 3',5'-monophosphate (cyclic GMP), or several of their derivatives produced little or no change in membrane potential. Prostaglandin E1 did not block the generation of postsynaptic potentials. Theophylline produced membrane effects that were different from those associated with postsynaptic potential generation; it also reduced the slow excitatory postsynaptic potential (EPSP) and potentiated the slow inhibitory postsynaptic potential (IPSP). The administration of papaverine, however, reduced both the slow EPSP and the slow IPSP. Although synaptic stimulation increases both cyclic GMP and cyclic AMP in these neurons, these results raise the possibility that these cyclic nucleotides may have functionla roles other than mediation of synaptic potentials.

Effect of phosphodiesterase inhibitors and dBcAMP on the inotropic and relaxing actions of histamine in cardiac muscle

The influence of phosphodiesterase inhibitors, imidazole and dBcAMP on the action of histamine on peak tension, on rate of tension development and rate of relaxation of ventricular muscle of guinea pig's heart was investigated. It was found that theophylline plus dBcAMP enhanced the peak tension, (dF/dt)max and the rate of relaxation and increased the relaxing action of histamine. Imidazole, an activator of phosphodiesterase, also potentiated the action of histamine on peak tension, (dF/dt)max and rate of relaxation. Caffeine, a well-known inhibitor of phosphodiesterase, suppressed the relaxing action of histamine and epinephrine. These discrepant results could be explained by assuming the possibility that these drugs have effects other than those involving inhibition and activation of phosphodiesterase.

Action of methylxanthines and imidazole on the contractility of the terminal ileum of the guinea pig

Methylxanthines (aminophylline and caffeine) and imidazole, substances with an opposite action on phosphodiesterase (PDE), were found to contract the terminal ileum and to potentiate nerve-mediated responses--contractions elicited by electrical stimulation (ES) at 3 Hz and 30 Hz. Imidazole-induced contractions which were partly reduced by atropine, potentiated both responses to ES to about the same extent, and enhanced contractility of the preparation to histamine and potassium. The action of imidazole on the terminal ileum could be related to its influence on PDE in the smooth muscle. The effects of aminophylline and caffeine were found to be more complex, possibly involving some mechanisms other than inhibition of PDE. They produced atropine-sensitive contractions of the terminal ileum, which were potentiated by physostigmine and strongly depressed by hemicholinium. In the presence of atropine, they potentiated ES-induced contractions, particularly those elicited by ES at 30 Hz, which are thought to be of adrenergic origin. Both actions appeared to be due to presynaptic effects -- activation of cholinergic and adrenergic neurons in the intestinal wall, possibly by enhanced influx of calcium, and facilitated release of acetylcholine and noradrenaline. Aminophylline, in concentrations which potentiated nerve-mediated contractions elicited by ES, did not affect direct smooth muscle-contracting action of drugs. Higher concentrations of aminophylline, above 0.1 mM, were found to inhibit histamine- and noradrenaline-induced contractions presumably due to inhibition of PDE in the smooth muscle and subsequent elevation of cAMP levels.

Possible role of cyclic AMP in the relaxation process of mammalian heart: effects of dibutyryl cyclic AMP and theophylline on potassium contractures in cat papillary muscles

The effect of dibutyryl cyclic AMP (DB-c-AMP; 3 X 10(-4)-3 X 10(-3) M) on electrically induced twitch and high potassium (142.4 mM KCl)-induced contracture tension was studied in papillary muscles from normal and reserpinized cats ([Ca]0 1.8 mM; 25 degrees C; pH 7.4). In both groups of preparations, the increase in twitch tension evoked by DB-c-AMP was accompanied by an abbreviation of the time to peak force and of relaxation time. In the same preparations, the high potassium contracture was markedly depressed by DB-c-AMP in a concentration-dependent manner. Similar results were obtained with the N6-monobutyryl derivative of cyclic AMP. The relaxing effects of the cyclic nucleotides on KCl contractures did not appear to be due to possible non-cyclic breakdown products: adenosine, 5'-AMP and sodium butyrate did not attenuate contracture tension at concentrations up to 3 X 10(-3) M. The same applies to ATP and non-cyclic N6-2'-0-3'-0-tributyryl-adenosine-monophosphate. Theophylline (10(-2) M) was found to prolong the relaxation time of the twitch and to enhance the high KCl contracture. It is concluded that cyclic AMP may be capable of modulating the relaxation process of mammalian heart and that not only the positive inotropic but also the relaxant effects of catecholamines on myocardium described before may be mediated by the cyclic AMP system. The relaxant effects of cyclic AMP derivatives on intact myocardial preparations are attributed to a stimulation by cyclic AMP of the calcium transport of the sarcoplasmic reticulum (SR) and are interpreted to be a corollary to the effects of cyclic AMP previously obtained on isolated SR preparations.

Effect of theophylline on calcium exchangeability in ventricular myocardium

The effects of theophylline on contractile force and myocardial calcium exchangeability were studied in isolated, electrically driven Langendorff perfused guinea-pig hearts. Following a 30-min exposure to 45Ca, total cellular calcium and 45Ca activity were measured in right ventricular samples. "Nontoxic" theophylline concentrations (5 x 10(-5) -10(-3) g/ml) which augmented contractile force without producing arrhythmias or contractures had no effect on total tissue calcium and did not alter the size of the fraction of cellular calcium exchangeable under steady-state conditions. A "toxic" concentration of theophylline (2 x 10(-3) g/ml) induced contractures and increased the amount of exchangeable cellular calcium. The latter effect was due to an increase in total calcium; the unlabelled cellular calcium fraction remained unchanged under the influence of all theophylline concentrations studied. The results suggest that theophylline increases the steady-state calcium exchangeability in ventricular myocardium only when the total calcium concentration is also increased.

Potentation of the cardiac response to aminophylline by oxyfedrine

The influence of oxyfedrine on the cardiostimulatory effects of aminophylline was studied in the isolated perfused guinea-pig heart. It was found that oxyfedrine potentiated the stimulatory effects of aminophylline on isometric contraction, dF/dt, coronary flow and heart rate. This potentation was abolished after pretreatment with propranolol. Histamine, though to a lesser extent, also potentiated the effects of aminophylline. When oxyfedrine and histamine were infused simultaneously in the presence of propranolol, the response of the heart to aminophylline was also potentiated; the magnitude of this potentiation was comparable to that obtained with histamine alone, indicating that propranolol abolished only the action of oxyfedrine but not that of histamine. The mechanical effects of aminophylline were accompanied by a slight (15 per cent) but significant inhibition of phosphodiesterase, which was not further augmented by oxyfedrine. The results suggest that the potentiating effects of oxyfedrine or histamine on the cardiostimulatory actions of aminophylline are elicited by their stimulatory actions on adenylate cyclase activity.

Relationship between theophylline uptake and inotropic effect in the guinea-pig heart

1 The time course of the positive inotropic effect of theophylline was compared with the time course of the uptake and release of [(3)H]-theophylline in guinea-pig isolated, electrically driven hearts perfused by the Langendorff method.2 Formation of theophylline metabolites could not be detected under the experimental conditions used.3 Theophylline entered myocardial tissue very rapidly in two different phases. The first process (half-time 21 s) amounted to 93% and the second (half-time 5 min 50 s) to 7% of the total uptake. The development of the positive inotropic effect of theophylline was about four times faster than even the rapid component of the uptake of the drug into the myocardium.4 The amount of theophylline accumulated in myocardial tissue (after 10 min perfusion) incerased proportionally with theophylline concentrations in the perfusion media and no signs of saturation were detected. The tissue-medium ratio did not exceed 1. The water content of the myocardial tissue amounted to about 80% at all theophylline concentrations examined.5 The uptake of theophylline (3 mg/ml) was diminished by 9.2% after pretreatment of the hearts with caffeine (1 mg/ml). Theophylline uptake was also decreased by 13.5% when caffeine-pretreated hearts were perfused with a solution containing theophylline (300 mug/ml) plus caffeine (1 mg/ml).6 Theophylline release from the hearts was also very rapid. The efflux curve was composed of three components (half-times: 24 s; 1 min 24 s; 6 min 18 seconds). The intercepts of the linear portions of the efflux curve occurred at 61%, 38% and 1%, respectively. Contractile force and theophylline content in myocardial tissue declined in a similar manner.7 It is concluded that theophylline enters myocardial tissue very rapidly by passive diffusion. Theophylline distributes itself in the heart as freely as in the perfusion medium. A very small amount may be bound within the cell in a relatively specific way.8 It seems possible that the positive inotropic effect of theophylline is partly due to an action of the drug on intracellular calcium binding or storage sites. However, the principal action of theophylline is assumed to be on the sarcolemma where it increases calcium influx from the extracellular space. This conclusion is based on the fact that the time courses of the increase in contractile force and of theophylline uptake into the cell were dissimilar.