Export of cyclic AMP from avian red cells. Independence from major membrane transporters and specific inhibition by prostaglandin A.

Prostaglandin A1 (PGA1) inhibits energy-dependent cyclic AMP export by pigeon red cells [Brunton and Mayer, J. Biol. Chem. 254:9714 (1979)]. To assess the specificity of this action, we observed the effect of 10 microM PGA1 (a concentration that inhibits cyclic AMP efflux greater than 95%) on a variety of membrane-protein-mediated processes that we could readily characterize and quantify in the pigeon red cell. Included in this study were isoproterenol-sensitive cyclic AMP production, ouabain-inhibitable 86Rb+ influx, furosemide-sensitive NaCl-KCl symport, 4-acetamido-4'-isothiocyano-2, 2'-disulfonic stilbene (SITS)-sensitive sulfate exchange, Na+-dependent alpha-aminoisobutyrate influx, and glucose and adenosine uptake. Remarkably, none of these membrane activities is significantly affected by PGA1. Furthermore, SITS, nitrobenzylthioinosine, cytochalasin B, and Na+-free extracellular medium (inhibitors of band 3, the nucleoside transporter, the hexose transporter, and amino acid uptake, respectively), failed to inhibit cyclic AMP export by pigeon red cells. On the basis of this data, we conclude that PGA1 does not act via a generalized alteration of a basic property at the plasma membrane, such as its fluidity; rather, PGA1 acts specifically to inhibit cyclic AMP extrusion. The data also imply that a transporter not relying on the Na+ gradient and distinct from transporters of cations, anions, amino acids, sugars, and nucleosides mediates cyclic AMP export.

A glutathione adduct of prostaglandin A1 acts intracellularly to elevate cyclic AMP by inhibiting its extrusion.

Prostaglandin A1 inhibits the energy-dependent export of cyclic AMP from avian red cells. When exposed to PGA1, avian red cells rapidly accumulate the prostaglandin and convert it to a polar metabolite. Inhibition of cyclic AMP efflux correlates with the intracellular content of this metabolite. Data in the literature and our chemical and chromatographic analyses indicate that the prostaglandin metabolite is a glutathione adduct of PGA1. Thus, we hypothesize that a GSH adduct of PGA1 acts intracellularly to elevate cyclic AMP by blocking its extrusion.

Taurine decreases lesion severity in the hearts of cardiomyopathic hamsters.

Cardiomyopathic Syrian hamsters develop necrotic lesions consequent upon calcium overload from 60 days of age onward. Taurine, given as a 0.1 M solution in place of drinking water for one month prior to sacrifice of animals of initial age 35 days, decreased the severity of subsequently developing cardiac lesions by 40%. Calcium concentration in the heart was decreased by 57%. Magnesium and iron concentrations were unaltered. Taurine given in a similar manner for 4 months had a protective effect, decreasing lesion severity by 21% and calcium concentration by 35%. Magnesium concentrations were increased by 12%. Compared to random-bred animals, cardiomyopathic hamsters at one and two months of age have the same concentrations of calcium, magnesium and iron in the quadrants of the heart, except in the left ventricle, which has significantly higher concentration of calcium. Calcium concentrations are 70%, 1320% and 2100% higher respectively in one month, two month and five month old animals. Five month old animals differ slightly but significantly in iron (17% decrease) and magnesium concentrations (17% increase). Cardiomyopathic hamsters have insignificant differences in beta-adrenergic receptor density compared to random-bred animals and have a significantly higher rate of taurine influx.

Differential effects of amrinone on contractility and taurine influx in rat and guinea pig hearts.

Amrinone (5-amino-3,4'-bipyridin-6(IH)-one) is a non-glycoside, non-catecholamine, positive inotropic agent with an unknown mechanism of action. In the Langendorff-perfused isolated guinea pig heart, we found that amrinone produced a maximum increase in contractile force of 33% at a concentration of 10 micrograms/ml (10.7 x 10(-5) M), without change in heart rate. Maximum response occurred within 2 min of initiating perfusion, and increased contractility persisted for several minutes of drug-free washout. Amrinone neither alleviated nor aggravated spontaneous arrhythmias. At the time of maximum inotropic response, amrinone produced no change in cyclic AMP levels. In contrast to the guinea pig, the Langendorff-perfused isolated rat heart responded with a decreased contractility when perfused with amrinone. Furthermore, whereas amrinone stimulated the influx of taurine in guinea pig hearts, taurine influx remained unaltered in rat hearts. This is the first case of stimulated taurine influx not being associated with an increase in cAMP concentrations. In the guinea pig heart, amrinone increased markedly the half life of exchange of intracellular calcium. It has been suggested that amrinone is positively inotropic because of a direct action on the contractile proteins. On the basis of our observations, we think it more likely to be due to the alterations in calcium flux caused by amrinone.

Physiological and experimental regulation of taurine content in the heart.

High concentrations of taurine are found in the heart and these are increased still further in congestive heart failure. It appears that taurine is largely derived by influx from the circulation, and this influx is stimulated by cyclic AMP, whereas influx of alpha-amino acids is unaffected. Influx occurs via a saturable transport system that has strict requirements for ligands. Other substances are transported by this system, including beta-alanine, hypotaurine, guanidoethyl sulfonate, and, to a lesser extent, guanidinopropionate; and these are competitive antagonists for taurine transport. Guanidinoethyl sulfonate, in vivo, markedly lowers taurine concentrations over the course of a few days in all tissues examined in the rat and mouse (but not in the guinea pig). The concentrations of other amino acids are unaffected. Guanidinoethyl sulfonate may prove to be a useful substance in the study of the biological role of taurine, in view of its ability to regulate taurine content in a number of species. Despite the numerous pharmacological actions of taurine, its physiological function in the heart remains problematic. One function appears to be the modulation of calcium movements. The inotropic actions of taurine and beta-adrenergic activation may be linked via the cyclic AMP-dependent regulation of taurine influx.

The mechanism of the adrenergic stimulation of taurine influx in the heart.

The mechanism of isoproterenol-stimulated taurine uptake was examined in the perfused rat heart. Hearts were perfused by the Langendorff technique in a non-recirculating system, while heart rate and contractile force were determined continuously. Perfusion with inotropic concentrations of glucagon stimulated the uptake of [3H]taurine. If the positive inotropic response to isoproterenol was blocked with verapamil, a calcium antagonist, the uptake of taurine was still stimulated. This indicates that inotropy per se, or calcium influx are not involved in the modulation of taurine influx, but that influx rats is responding to cell cyclic AMP levels. The lack of effect of the positively inotropic ionophores monensin and A23187 and the negatively inotropic ionophore valinomycin is in agreement with this conclusion. Taurine decreased calcium binding to cardiac sarcolemma by 31% at 1 mM concentration and 80% at 5 mM.

The Syrian golden hamster: a model for the cardiomyopathy of Friedreich's ataxia.

In light of the available information on the cardiomyopathy of Friedreich's ataxia, the cardiomyopathic Syrian hamster may be an appropriate laboratory model. Cardiomyopathy in these animals is a result of calcium accumulation. We analyzed the atria and right and left ventricles from cardiomyopathic (CM) and random bred (RB) animals for calcium, magnesium, and iron concentrations at 30-40 and 60-70 days of age (age of maximum lesioning). There are no significant differences in the concentration of iron or magnesium among age-matched groups. The concentration of calcium in the left ventricles of the CM animals at 60 days old is 14 fold higher than that of RB animals. Although there is a significant difference in the concentration of calcium in the left ventricles of younger animals, it is not as pronounced as the difference in older animals. Analysis of the taurine concentration in 30-40 day old animals revealed that the CM animals show slightly higher taurine concentrations than RB in the whole heart. In 60 day old CM hamsters in the beta-adrenergic receptor density of the ventricles is unchanged. This indicates that calcium overload is not due to adrenergic supersensitivity.

Brain neurotransmitter receptors in Friedreich's ataxia.

The binding of 3H-quinuclidinyl benzilate, a muscarinic cholinergic antagonist, of 3H-dihydroalprenolol, a beta adrenergic antagonist, and of 3H-flunitrazepam, a ligand which labels benzodiazepine receptors, was examined in several regions of control and Friedreich's ataxia (FA) brains. 3H-Quinuclidinyl benzilate binding appeared to increase in the inferior olivory nucleus, anterior and posterior cerebellar vermi but was unaltered in the dentate nucleus and cerebellar hemisphere of FA brain. The binding of 3H-dihydroalprenolol seemed to increase in the inferior olivary nucleus yet was not different from controls in the dentate nucleus, cerebellar hemisphere, anterior and posterior cerebellar vermi of FA brains. 3H-Flunitrazepam binding was slightly lowered in the inferior olivary and dentate nuclei but was unchanged in the other FA brain regions examined. The present study suggests possible trends in neurotransmitter receptor alterations in post-mortem brain tissue of FA patients.

Regional distribution of amino acids in Friedreich's ataxia brains.

The distribution of amino acids in two Friedreich's ataxia brains is compared to 4 control brains. Glutamate and GABA were decreased in the cerebellar hemispheres and/or in the vermis. Taurine concentrations were uniformly elevated throughout the areas studied. Taurine/glutamate and taurine/GABA ratios were consistently elevated in Friedreich's ataxia brains, whereas glutamate/glutamine ratios were consistently decreased (with normal glutamine concentrations).