Lack of interaction between norepinephrine or histamine and theophylline on cardiac cyclic AMP

Studies on the mechanism of action of the bipyridine milrinone on the heart

Milrinone is a positive inotropic and vasodilator agent when tested in experimental animals and in human heart-failure patients. It is generally believed that milrinone acts by inhibiting phosphodiesterase IV, thus increasing cyclic AMP, [Ca++]i and cardiac contractile force and relaxation. Maximal force produced by milrinone is greater when single-dose response curves are compared to cumulative dose-response curves. In vitro, milrinone produces a tachyphylaxis, the extent of which is both dose- and time-dependent. Recovery of tachyphylaxis is both dose- and time-dependent and is not influenced by inhibitors of protein or RNA synthesis. There is a specific cross-tachyphylaxis between milrinone and amrinone, theophylline, papaverine, and Bay K8644. This tachyphylaxis may explain the low maximal contractile response of the cumulative dose-response observed in isolated tissues. Milrinone increased cyclic AMP in dog and guinea pig cardiac muscle. As previously shown by Endoh et al., milrinone in low doses produced a biphasic effect on cyclic AMP. The early increase (first 60-70 s) in cyclic AMP shows a good correlation with contractile force changes. If cyclic AMP is determined at maximal contractile force this correlation was poor. Here we also present instances where the increase in cyclic AMP after milrinone (determined at maximal effect) does not correlate with the contractile response. The cross-tachyphylaxis of milrinone with Bay K8644 suggests that milrinone has an action on the sarcolemmal Ca++ channels. Bay K8644 suppresses the positive inotropic effect of catecholamines by 50%, but not the cyclic AMP response. The inotropic effect of milrinone, in contrast to norepinephrine is highly sensitive to [Ca++]0, stimulation rate, and [K+]0. In this respect milrinone behaves more like Bay K8644. We postulate that the main inotropic action of milrinone is due to a sarcolemmal effect. The early cyclic AMP production described could be in the sarcolemmal compartment and this may explain some of the similarities of milrinone's actions with those of Bay K8644. The tachyphylaxis observed with the inotropic effect of milrinone does not extend to the decreases in relaxation time. This and other findings to be discussed suggest that the positive inotropic and reduction in relaxation time by milrinone depend on different mechanisms, possibly through differential compartmentalization of cyclic AMP.

Antagonistic effect between histamine and norepinephrine on isolated guinea-pig right atrium

Norepinephrine (NE) and histamine (His) cause a dose related increase in the rate of the guinea pig isolated right atria. The rate increase induced by both amines has been shown to be mediated by pharmacologically different receptors. His shifted the NE CEC to the right in a non parallel fashion. Cimetidine (10(-4) mol/l) abolished this antagonism. Norepinephrine (3 X 10(-7) mol/l) shifted to the right the His CEC also in a non parallel fashion. Practolol (10(-4) mol/l) abolished this antagonism. Theophylline (10(-4) mol/l) reverted both: the antagonistic action of His related to the NE chronotropic effect and the antagonistic action of NE related to the His chronotropic effect. The reversal of the antagonism could be the result of the block of purinergic P1-receptors by theophylline. This block impairs the chronotropic effect of adenilic compounds which can function as a modulater of cardiac rate through a negative feed back mechanism.

Effect of cyclic AMP, db-cyclic AMP and phosphodiesterase inhibitors on histamine inhibition of the contractile response of the mouse vas deferens

Histamine inhibited the contractile response of the mouse vas deferens to various frequencies of stimulation, and the inhibition was inversely related to the frequency of stimulation. This effect of histamine was mimicked by cyclic AMP, db-cyclic AMP and various phosphodiesterase inhibitors (IBMX, aminophylline and theophylline). Histamine-produced inhibition, but not that produced by the other compounds, was blocked by cimetidine. The low concentrations (10-100 microM) of various phosphodiesterase inhibitors caused inhibition but failed to potentiate the inhibitory response to histamine. The basal cyclic AMP levels of the tissues were unaffected at these concentrations of aminophylline. At higher concentrations (1 and 5 mM), however, aminophylline significantly elevated the basal cyclic AMP levels of the tissues and markedly inhibited the contractile response to various frequencies of stimulation but still failed to enhance or potentiate the inhibitory response to histamine. In fact, the inhibitory response to histamine at these concentrations of aminophylline was reduced. Based on these and earlier [3] findings, it is concluded that, although histamine increases the accumulation of cyclic AMP in the mouse vas deferens and although its inhibitory effect on the preparation can be mimicked by both the cyclic nucleotides and phosphodiesterase inhibitors, the involvement of cyclic AMP in the mediation of its inhibitory response is still unresolved.

The effects of theophylline and 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone (RO 20-1724) on protein secretion from rat parotid gland

1 The effects of two chemically distinct cyclic nucleotide phosphodiesterase (PDE) inhibitors on protein secretion from superfused rat parotid gland were studied.2 In the presence of 1.0 mM Ca(2+), Ro 20-1724 (10 muM), an imidazolidinone derivative, increased the secretory response to isoprenaline 100% and the isoprenaline-dependent accumulation of adenosine cyclic 3',5'-monophosphate (cyclic AMP) 300-400%. At this concentration Ro 20-1724 alone did not cause protein secretion, accumulation of cyclic AMP or significantly inhibit PDE activity in cell-free preparations from parotid gland.3 In the absence of added Ca(2+) and in the presence of 1.0 mM EGTA, Ro 20-1724 inhibited the secretory response to isoprenaline 65% while increasing isoprenaline-dependent cyclic AMP accumulation 200%.4 In the presence of Ca(2+), theophylline (10 mM) stimulated protein secretion but did not cause the accumulation of cyclic AMP. When combined with isoprenaline the rate of secretion was greater than the sum of the effects of the individual drugs but there was no effect of theophylline on the isoprenaline-dependent accumulation of cyclic AMP.5 Theophylline-stimulated protein secretion is increased by omitting Ca(2+) from the superfusion medium without any detectable change in cyclic AMP accumulation. Under these conditions Ro 20-1724 inhibits theophylline-stimulated protein secretion and the maximum rate of protein secretion in the presence of isoprenaline and theophylline is no greater than that seen with either agent alone.6 It is concluded that the theophylline effects do not result from inhibition of PDE. It is suggested that the primary action of both drugs on parotid gland acinar cells is to alter the distribution of intracellular Ca(2+). Ro 20-1724 may also inhibit Ca(2+)/calmodulin activated enzymes such as PDE.

The effect of phosphodiesterase inhibitors on guinea-pig cardiac responses to histamine and isoprenaline

(1) The effect of phosphodiesterase inhibitors was studied on right artial rate, left atrial tension and left ventricular papillary muscle tension responses to histamine and isoprenaline. (2) Only responses mediated via beta-adrenoceptors and H2-receptors were potentiated by theophylline. This is proposed to be due to its phosphodiesterase inhibiting properties and therefore indicates the involvement of cAMP in these responses. (3) 3-Isobutyl-1-methylxanthine was approximately 30 times more potent than theophylline in producing leftward shifts of isoprenaline left atrial tension curves. The potentiating effect of papaverine was masked by an opposing depressant action. (4) Left ventricular papillary muscle tension responses to histamine were enhanced by the phosphodiesterase inhibitors confirming the reported involvement of cAMP in the right ventricle. (5) The left artial tension dose-response curves to histamine were not potentiated. Single doses revealed a biphasic response consisting of an H1-receptor mediated component and one resistant to both H1- and H2-receptor anagonists. Neither component was potentiated suggesting no involvement of cAMP.

Influence of thyroid hormone on the positive inotropic effects mediated by alpha- and beta-adrenoceptors in isolated guinea pig atria and rabbit papillary muscles

Effects of thyroxine treatment for 7--11 days on the positive inotropic effects mediated by alpha- and beta-adrenoceptors were studied in isolated guinea pig atria and rabbit papillary muscles. In guinea pig atria, the thyroxine treatment inhibited the positive inotropic effect of lower concentrations of phenylephrine (PHE), and attenuated the inhibitory effect of phentolamine on the PHE response. The effect of isoproterenol (ISO) was potentiated by the thyroxine treatment. In rabbit papillary muscles, the thyroxine treatment shifted the dose--response curve for PHE to the right and attenuated the inhibitory effect of phentolamine on the PHE response. Propranolol, in both guinea pig atria and rabbit papillary muscles, inhibited the PHE response more effectively in preparations from thyroxine-treated animals than in controls. In guinea peg atria, the attenuation of the PHE response mediated by alpha-adrenoceptors was observed after the thyroxine treatment for only 2 days, whereas the potentiation of the ISO response required the thyroxine treatment for a longer period. It was concluded that the thyroxine treatment attenuated the positive inotropic effect mediated by alpha-adrenoceptors and potentiated that mediated by beta-adrenoceptor-mediated positive inotropic effects due to the thyroxine treatment may be independent of each other.

The relationships between cyclic AMP, calcium and prostaglandins as second messengers

There is considerable dissatisfaction with present second messenger hypotheses involving cyclic nucleotides and calcium. The recent findings that methyl xanthines and adenosine are prostaglandin antagonists casts doubt on much of the evidence in favour of the cyclic AMP hypothesis. There is evidence that allosteric sites may modify the binding of calcium and cyclic nucleotides to key cellular regulators and that a range of substances including steroids, adenosine and prostaglandins may occupy those sites. This concept introduces much needed flexibility into the second messenger concept, allows many experiments to be reinterpreted and has major implications throughout the biomedical sciences.

The effects of methylxanthines on catecholamine-stimulated and normal chick embryos

Dose of theophylline and caffeine which do not produce aortic arch anomalies in embryonic chicks have been shown to potentiate catecholamine-induced aortic arch malformations in that experimental animal. Theophylline (2.1 X 10(-5) mole per milliliter isotonic saline solution) potentiated the effective dose of norepinephrine more than 100 times. The greatest potentiation observed with epinephrine (2.5 X) was induced by 2.6 X 10(-5) mole caffeine. This study also demonstrated that both methylxanthines specifically induce aneurysms of the ascending aorta and complete absence (or nearly complete constriction) of the right ductus arteriosus. The incidences of these types of cardiovascular malformations proved to be dose dependent with theophylline a more potent teratogen than caffeine. The mobilization of calcium and/or cyclic nucleotide phosphodiesterase inhibition by the methylxanthines are suggested as significant actions in the potentiation of catecholamine-induced aortic arch anomalies.

Theophylline and phenylephrine effects on cardiac relaxation

In the driven isolated left atrium of the rabbit theophylline shortened relaxation time in a similar manner to isoprenaline and histamine. 2 Phenylephrine lengthened relaxation time in a similar manner to calcium. 3 Theophylline caused phenylephrine to shorten relaxation time, which was inhibited by a beta-adrenoceptor blocking drug, but theophylline did not potentiate the effect of phenylephrine on peak tension. 4 Theophylline separated drug effects on cardiac relaxation and contraction: in the presence of theophylline at a low calcium concentration, phenylephrine shortened relaxation time by beta-adrenoceptor stimulation and increased peak tension by alpha-adrenoceptor stimulation. At a high calcium concentration, theophylline potentiated the effect of isoprenaline, histamine and phenylephrine on relaxation time but inhibited the effect on peak tension.

Relationship between the level of cAMP and the contractile force under stimulation of alpha- and beta-adrenoceptors by phenylephrine in the isolated rabbit papillary muscle

The time course of changes of the level of 3',5'-cyclic AMP (cAMP) and of the tension developed under stimulation of alpha- and beta-adrenoceptors by phenylephrine was investigated in the isolated rabbit papillary muscle. Furthermore the dose-response relationships for increases of cAMP and of developed tension elicited by phenylephrine were determined. 1. A submaximally effective concentration of phenylephrine (10(-5) M) increased significantly the level of cAMP of the papillary muscle at 15 and 30 s by 45 and 36% respectively; the level of cAMP returned to the control value at 60 s after the administration. The developed tension increased significantly not before 45 s and reached its maximal level at 180 s. 2. When alpha-adrenoceptors were blocked by phentolamine (10(-6) M), the positive inotropic effect of phenylephrine was decreased significantly but the increase of cAMP induced by phenylephrine was not reduced. In the presence of phentolamine the increase of cAMP induced by phenylephrine lasted longer than in the control experiments. 3. The effects of phenylephrine (10(-5) M) both on the level of cAMP and the developed tension mediated via stimulation of beta-adrenoceptors in the presence of phentolamine were enhanced by the phosphodiesterase inhibitor papaverine throughout the course of responses. 4. Phenylephrine produced an increase in developed tension as well as in cAMP. The corresponding dose-response curves run parallel to each other but differed by about 1.5 log units whereby the developed tension was evoked by lower concentrations. Phentolamine (10(-6) M) shifted the curve for the positive inotropic action by about 1.5 log units but did not affect that for increase in cAMP. Therefore, in the presence of the alpha-adrenolytic drug phentolamine the difference between both curves became smaller so that both curves were superimposed. Papaverine (10(-5) M) shifted the whole curve for cAMP upwards and enhanced the maximal contractile response to phenylephrine mediated by stimulation of beta-adrenoceptors. 5. The present results indicate that the positive inotropic action of phenylephrine in lower concentrations (less than 10(-5) M) induced by stimulation of alpha-adrenoceptors is independent of the level of cAMP. The positive inotropic action of the higher concentrations of phenylephrine induced via stimulation of beta-adrenoceptors was preceded by an accumulation of cAMP; the inhibition of the cAMP phosphodiesterase activity by papaverine enhanced the actions of phenylephrine both on the level of cAMP and on the contractile force.