The effect of triamterene on myocardial phosphodiesterase and adenylate cyclase

The study concerned the effect of triamterene (TA) and its phase-II metabolite p-hydroxytriamterene-sulphuric acid ester (OH-TA ester) on myocardial phosphodiesterase (PDE) and adenylate cyclase (AC): (1) TA and OH-TA ester exerted a concentration-dependent inhibition of PDE. The estimated IC50 values of 127 and 72 mumol/l were 16 and 9 times higher than that of papaverine with 8 mumol/l. At comparable concentrations, TA produced its inotropic effect (EC50 96 mumol/l) in isolated guinea-pig atria so that PDE inhibition may well account for its pharmacodynamic action. (2) In isolated guinea-pig atria, the cyclic AMP content increased significantly with 15-30 s after the addition of TA and remained increased over a period of 15 min. This increase preceded the positive inotropic action, which reached its maximum after 5-10 min. (3) TA did not affect the basal activity of myocardial AC preparations. AC activation by isoprenaline (30 mumol/l) was, however, strongly inhibited.

The interaction of triamterene at the myocardial beta receptor site

Triamterene (TA) inhibits in the microM range, as was previously shown in our laboratory, the positive inotropic action of beta-adrenoceptor agonists like isoproterenol in atrial preparations, when applied first. This interaction may be best explained by a direct influence of TA on the beta-adrenergic receptor-site itself or by an uncoupling of the signal transfer between receptor and adenylate cyclase complex at the inner sarcolemmal membrane site. For clarification we performed biochemical studies at freshly prepared membrane particles from guinea-pig hearts, combining both hypothetical sites of interaction. Binding studies with [3H]-dihydroalprenolol to the myocardial beta-adrenoceptor revealed that TA (1 to 10 mumol/l) did not disturb their functioning excluding a possible interaction at this membrane site. On the other hand we found a non competitive inhibition of the isoprenaline-activated adenylate cyclase while basic activity was not altered in the presence of triamterene (TA, 5 to 200 mumol/l). From the concentration-response curves a Ki-value of TA for half-maximal inhibition of 2.8 mumol/l was calculated. Since the sodium fluoride activated adenylate cyclase was also inhibited by triamterene in the same concentration range we conclude that it interferes with the signal-transfer between the beta-adrenoceptor site and the adenylate cyclase system through the plasmalemma.

Comparison of the effects of theophylline, prednisolone and sleep withdrawal on airway obstruction and urinary cyclic AMP/cyclic GMP excretion of asthmatic children with and without nocturnal asthma

Asthmatic children suffering from nocturnal asthmatic paroxysms showed not only a reduced peak exspiratory flow rate (PEER) but also a diminished excretion of cyclic AMP (cAMP) at night. In contrast, in asthmatic children without nocturnal asthmatic attacks, neither in PEFR nor in cAMP excretion did a circadian rhythm occur. Prednisolone, theophylline, and sleep withdrawal were able to reestablish normal airway function and cAMP excretion in those patients with nocturnal bronchoconstriction. Cyclic GMP (cGMP), which has been suggested as a mediator for cholinergic mechanisms, shows the exact opposite behavior of that of cAMP. In children suffering from nocturnal asthmatic attacks, cGMP increases during the night, whereas under sleep withdrawal it decreases. Hence we propose that a reduced responsiveness of beta-adrenergic receptors as well as an increased cholinergic activity are due to nocturnal asthmatic attacks. Therapeutic interventions taking the form of increasing cAMP or decreasing cGMP levels may thus be most helpful in treating nocturnal asthma.

Central blood pressure effects of guanylyl-imido-diphosphate and cyclic guanosine-monophosphate

Injections of guanylyl-imido-diphosphate (250, 500 and 1,000 microgram/kg) into the lateral cerebral ventricle of the anaesthetized cat induced increases in blood pressure and heart rate while the intravenous injections of the same doses were ineffective, thus indicating a central mechanism of action of this compound which activates adenylcyclase at the catalytic subunit. The results support the hypothesis that the activity of cardiovascular centres depends on the prevailing concentration of cAMP. Intracerebroventricular injection of cGMP (125, 250 and 500 microgram/kg) caused hypotension and bradycardia. The effects increased with the dose but were subject to tachyphylaxis. The lack of an effect after intravenous administration indicates a central site of action. This result is in agreement with the Yin Yang hypothesis and suggests that cGMP is a second transmitter in cardiovascular centres which may be involved in central cardiovascular effects in response to stimulation by putative neurotransmitter substances such as acetylcholine.