Negative inotropic effect of vanadate in ventricular myocardium in the presence of 3-isobutyl-1-methylxanthine or isoprenaline

Adenosine inhibits the positive inotropic effect of 3-isobutyl-1-methylxanthine in papillary muscles without effect on cyclic AMP or cyclic GMP

1 Adenosine and the adenosine receptor agonist (-)-N6-phenylisopropyladenosine (PIA) produced a small positive and negative inotropic effect, respectively, in isolated electrically driven papillary muscles of guinea-pigs. 2 Adenosine (100 mumol l-1) had no effect on cyclic AMP or cyclic GMP content. PIA (100 mumol l-1) slightly increased cyclic AMP. 3 In the presence of 3-isobutyl-1-methylxanthine (IBMX; 60 mumol l-1), which increased force of contraction 2 fold, adenosine and PIA exerted strong negative inotropic effects. PIA was more potent than adenosine (mean IC25 2.1 and 168 mumol -1, respectively). 4 In contrast, the nucleosides did not affect the increase in force of contraction produced by elevating extracellular Ca2+ concentration. 5 The IBMX-antagonistic effects of adenosine and PIA were not accompanied by modification of the IBMX-induced increase in cyclic AMP and cyclic GMP. 6 The effects of adenosine and PIA on force of contraction were accompanied by a partial reversal of the IBMX-induced increase in the maximal rate of depolarization of slow action potentials. 7 It is concluded that adenosine and PIA are able to attenuate the positive inotropic effect of a phosphodiesterase inhibitor. This effect is unlikely to be due to a reduction of the IBMX-induced increase in cyclic AMP content. It is conceivably due to an inhibition of the stimulant action of cyclic AMP on slow Ca2+ channels leading to the reduction of the slow inward current which in turn reduces force of contraction.

Role of guanine nucleotide-binding protein in the regulation by adenosine of cardiac potassium conductance and force of contraction. Evaluation with pertussis toxin

In atrial cardiac preparations adenosine exerts a receptor-mediated negative inotropic effect due to an increased potassium conductance. Pretreatment of guinea pigs with pertussis toxin abolished the negative inotropic and action potential shortening effect of adenosine and the adenosine analogue (-)-N6-phenylisopropyladenosine (PIA). As pertussis toxin specifically inactivates guanine nucleotide-binding proteins involved in the signal transfer from receptor binding to specific cell functions, it is concluded that a guanine nucleotide-binding protein is involved in the regulation of the receptor-mediated change in potassium conductance and force of contraction.

Evidence for adenosine receptor-mediated isoprenaline-antagonistic effects of the adenosine analogs PIA and NECA on force of contraction in guinea-pig atrial and ventricular cardiac preparations

The effects of the adenosine agonists (-)-N6-phenylisopropyladenosine (PIA) and 5'-N-ethylcarboxamideadenosine (NECA) on force of contraction, adenylate cyclase activity and normal as well as slow action potentials were studied in guinea-pig isolated atrial (left auricles) and ventricular preparations (papillary muscles). In auricles PIA and NECA exerted concentration-dependent negative inotropic effects with similar potencies (mean EC50:0.05 mumol l-1 for PIA and 0.03 mumol l-1 for NECA). Similar results were obtained in the presence of isoprenaline. In papillary muscles PIA and NECA alone had no effect on force of contraction but produced negative inotropic effects in the presence of isoprenaline (mean EC50:0.19 mumol l-1 for PIA and 0.10 mumol l-1 for NECA). In both preparations, the negative inotropic effects of PIA and NECA in the presence of isoprenaline were antagonized by the adenosine receptor antagonist 8-phenyltheophylline. In both preparations, PIA and NECA did not affect adenylate cyclase activity, both in the absence and presence of isoprenaline. In auricles the negative inotropic effects of both nucleosides were accompanied by a shortening of the action potential. This effect was also observed in the presence of isoprenaline. In papillary muscles the adenosine analogs did not detectably alter the shape of the normal action potential. Ca2+-dependent slow action potentials elicited in potassium-depolarized preparations also remained unaltered in the presence of PIA or NECA alone. However, the isoprenaline-induced enhancement of the maximal rate of depolarization of slow action potentials was attenuated by PIA or NECA.(ABSTRACT TRUNCATED AT 250 WORDS)

Isoprenaline-like effects of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine on mechanical, biochemical and electrophysiological parameters in the mammalian heart

The phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) mimicked the effects of isoprenaline on the force of contraction, the cAMP content and the slow Ca++ inward current (Isi) in isolated guinea pig papillary muscles. The results support the hypothesis that phosphodiesterase inhibitors and beta-adrenoceptor agonists exert their positive inotropic effects by increasing Isi via the common mediator cAMP.

The effects of vanadate on rabbit ventricular muscle adenylate cyclase and sodium pump activities

Vanadate in the +5 oxidation state has been reported to have a positive inotropic action on cardiac ventricular muscle. We have investigated the biochemical actions of vanadate on ventricular muscle adenylate cyclase and sodium pump activities in both intact or disrupted cell systems in an attempt to elucidate the mechanism(s) responsible for the physiological response. Vanadate at concentrations up to 100 microM (Ka = 2 microM) stimulated adenylate cyclase activity in sarcolemmal membrane preparations or disrupted myocytes isolated from rabbit ventricular muscle by 2-3-fold. Increasing the vanadate concentrations above 100 microM resulted in a progressive inhibition of basal or hormone-stimulated adenylate cyclase activity (Ki = 5 mM) which was similar to that found by the reaction product, pyrophosphate (Ki = 0.5 mM). Both activation and inhibition by vanadate was fully reversible. Maximum activation of adenylate cyclase by vanadate and isoprenaline were not additive whereas maximum fluoride activation was decreased (18%) and the forskolin-stimulated response was slightly potentiated. Vanadate reversibly inhibited ouabain-sensitive p-nitrophenylphosphatase activity (Ki = 60 nM) in sarcolemmal membrane preparations and disrupted myocytes. Complete inactivation was found at 1 microM vanadate. Acute or chronic incubation of intact myocytes with vanadate at concentrations up to 0.5 mM had no measurable affect on ouabain-sensitive 86Rb influx or isobutylmethylxanthine, isoprenaline or forskolin-stimulated accumulation of intracellular cAMP concentration. Inhibition of 86Rb influx and cAMP accumulation was found at higher concentrations of vanadate; however, this accompanied the progressive decrease in cell viability as measured by the decrease in percentage of rod-shaped cells. It is concluded that vanadate, at concentrations which have been reported to induce a positive inotropic action on mammalian ventricular muscle, does not increase adenylate cyclase activity or inhibit the sodium pump activity in intact myocytes. These results show that caution must be applied when extrapolating the actions found with vanadate in broken cell systems to intact tissues.

Minireview: physiological and pharmacological properties of vanadium

Vanadium is distributed extensively in nature. It is a trace element and is present in almost all living organisms including man. Even though vanadium was originally recognized for its ability to inhibit membrane Na+-K+-ATPase, various laboratory studies now document that this element has the capacity to affect the activity of various intracellular enzyme systems and may modify their physiological functions. Vanadium may be an essential element for normal development and may play an important role in various homeostatic mechanisms, and thus vanadium deficiency may prove to be an important concern. Abnormalities in biological disposition of vanadium may be involved in the pathogenesis of certain neurological disorders or cardiovascular diseases. While the essentiality of this element for living organisms is yet to be established with certainty, vanadium has become an increasingly important element and is used extensively in various heavy industries such as steel, oil, etc.; thus, the incidence of exposure to toxic levels of vanadium to industrial workers has been an increasing concern for toxicologists. To date, little information is available on the physiological or pharmacological actions of vanadium; hence, it is difficult to reach any definitive conclusion concerning its biological significance, essentiality and its role in pathological states. An attempt has been made in this review to broadly document what is known of various biological actions of vanadium.