Regulation of calcium homeostasis in the heart by taurine

The role of taurine in maintaining calcium and potassium homeostasis in excitable tissues is discussed. These effects of taurine appear to be related to its interaction with a low affinity binding protein of the cell membrane. This conclusion is based on the observation that the sulfinic acid analog of taurine, hypotaurine, also interacts with this protein and mimics these actions of taurine, whereas two other analogs, beta-alanine and isethionic acid, have little affinity for the binding protein and fail to exhibit taurine-like activity. The possibility that a relationship exists between the low affinity protein and sarcolemmal calcium pools is considered.

Localization of sensitive sites to taurine, gamma-aminobutyric acid, glycine and beta-alanine in the molecular layer of guinea-pig cerebellar slices

1 The localization, in the molecular layer of guinea-pig cerebellar slices, of the sites most sensitive to iontophoretically applied taurine, gamma-aminobutyric acid (GABA), glycine and beta-alanine was investigated. 2 The most sensitive sites were located (expressed as the distance from the Purkinje cell body to the pial surface): 0 micrometers, 60 micrometers and 220 micrometers for taurine; 0 micrometers and 180 micrometers for GABA; 80 micrometers and 200 micrometers for glycine, and 80 micrometers, 180 micrometers and 300 micrometers for beta-alanine. 3 The sensitive site (at 0 micrometers) to GABA was considered to represent the basket synapses on the Purkinje cell soma, while the sites (60 to 80 micrometers and 200 to 300 micrometers) for taurine, glycine and beta-alanine were tentatively assigned to the synapses of the stellate neurones on the Purkinje cell dendrites. 4 Inhibitory actions of all four amino acids tested at the most sensitive sites were antagonized by both picrotoxin and strychnine. 5 The possibility that taurine might be the neurotransmitter of the stellate neurones in guinea-pig cerebellum is discussed.

Role of taurine as a possible transmitter in the thermoregulatory pathways of the rat

Taurine (10 and 20 micrograms) injected unilaterally into the lateral ventricle of rats caused an increase in core temperature. Bilateral injection of taurine 2.5 and 5 micrograms into the preoptic region of the anterior hypothalamus induced a dose-related hyperthermia: higher doses (10 micrograms) caused hypothermia. Intrahypothalamically taurine-induced hyperthermia was blocked by prior injection of strychnine hydrochloride (5 and 15 micrograms); doses which alone had no effect on core temperature. Of the other inhibitory amino acids injected intrahypothalamically hypotaurine also induced a hyperthermia. GABA (10 micrograms) caused hypothermia; glycine (10 micrograms) had no effect. Potassium (50 mM) stimulated release of radioactivity from superfused slices of anterior hypothalamus prelabelled with [3H]taurine in a calcium-dependent manner. A high affinity uptake mechanism with a Km of 8.5 microM was demonstrated with [3H]taurine into slices of anterior hypothalamus. Taurine may have a neurotransmitter role in the anterior hypothalamus but whether the body temperature effects represent physiological or pharmacological events remains to be established.

Evidence of a neurotransmitter role for aspartate and gamma-aminobutyric acid in the rat olfactory cortex

1. Using a cortical cup technique, the release of seven endogenous amino acids from the isolated rat olfactory cortex slice has been monitored.2. Electrical stimulation of the lateral olfactory tract at a frequency of 4 min(-1) was accompanied by a significant increase in the release of aspartate, GABA and taurine; the release of GABA and aspartate but not that of taurine was Ca(2+)-dependent.3. Chronic unilateral bulbectomy was accompanied by a specific, significant fall in the aspartate content of the olfactory cortex which reached a maximum 5 days after surgery and persisted for at least a further 5 days. Electrical stimulation of the lateral olfactory tract of such preparations did not release any of the amino acids under investigation.4. When slices from the unoperated side were exposed to solutions containing 25 mM-K+ or stimulated with electrodes placed directly on the cortex enclosed by the cup there was a Ca(2+)-dependent release of aspartate and GABA accompanied by a Ca(2+)-independent release of taurine. Following chronic bulbectomy, these procedures failed to evoke significant release of aspartate whereas the characteristics of GABA and taurine release were unaltered.5. It is concluded that aspartate may be the excitatory transmitter of some of the terminals of the lateral olfactory tract fibres and that GABA may be a transmitter at some inhibitory synapses of the rat olfactory cortex.

Taurine: stimulation of pineal N-acetyltransferase activity and melatonin production via a beta-adrenergic mechanism

Pineal glands convert [3H]tryptophan to [3H]N-acetylserotonin and [3H]melatonin in organ culture. Taurine treatment increases the rate of production of these compounds 40- and 25-fold respectively by stimulating the activity of N-acetyltransferase. This stimulation is blocked stereospecifically by L-propranolol, indicating that taurine is probably acting via beta-adrenergic receptors. Taurine is active in stimulating N-acetyltransferase activity in denervated glands, suggesting that it might interact directly with the beta-adrenergic receptor, and not by causing the release of norepinephrine from nerve terminals.

Amino acids and cell regulation

Free amino play an important role in regulating cell volume in fishes. Four tissues/cells (skeletal muscle, RBC, brain, and myocardium) of the little skate, Raja erinacea, were selected for detailed study because of their special importance or unique advantage as experimental models. Three particular amino acids, beta-alanine, taurine, and sarcosine play a predominant role in all four tissues. As in higher vertebrates, amino acid uptake in skate brain, heart, and RBC is mediated via a Na+-dependent process. Amino acids leave the skate brain rapidly in response to a sudden decrease in plasma osmolality and/or to a simultaneous drop in extracellular Na+ concentration. However, although amino acids are important for volume regulation in normal brain cells, they do not appear to be likely candidates for the unidentified "idiogenic" osmolytes in mammalian brain cells. The high concentration of taurine in skate myocardium is of special interest because of the special role of this amino acid in myocardial contractility. Thus, unlike beta-alanine and sarcosine, taurine may play a dual role in regulating both cell volume and contractility of myocardial cells. The isolated skate atrium is well suited for in vitro studies of these two processes.

The effects of isoproterenol on taurine concentration in the rat heart

Administration of the beta-agonist isoproterenol to rats caused a sharp increase in the influx rate of taurine into the heart, but had no effect on the rate of biosynthesis of taurine in the heart. Influx rates of other amino acids were also stimulated, the distribution ratios, (dpm/g heart)/(dpm/ml plasma), of beta-amino acids being 3--4 times higher than alpha-amino acids. The doses of isoproterenol employed induced cardiac hypertrophy, but the stimulation of taurine influx appeared to be an independent phenomenon, in that near maximum stimulation of influx was achieved prior to stimulation of ornithine decarboxylase activity. Reserpine treatment also produced stimulation of taurine influx with no effect on biosynthesis. These results indicate a close connection between increased sympathomimetic activity and increased influx of amino acids.

Taurine and other free amino acids in milk of man and other mammals

Taurine and other free amino acids have been determined in human milk of a number of other species. Taurine is the most abundant free amino acid in the milk of the gerbil, mouse, cat, dog and rhesus monkey. Taurine is the second most abundant amino acid in the milk of the rat, baboon, chimpanzee, sheep, Java monkey and man. Taurine is not a major constituent in the milk of the guinea pig, rabbit, cow and horse. The milk of each species has a characteristic free amino acid pattern which may be an indication of the relative nutritional importance of these compounds during early postnatal development.

Calcium-dependent increase in efflux of [1-3H] taurine from the superfused rat cerebellar cortex in vivo

High (40 mM) potassium stimulation has been shown to increase the efflux in vivo of [1-3H] taurine from the superfused rat cerebellar cortex by 46%, P less than 0.001. During superfusion with calcium-free media this increase in efflux is abolished. The cellular location of [1-3H] taurine in the cerebellar cortex is not yet known, but this region of the brain contains exceptionally high levels of endogenous taurine. The calcium dependency of the release of labelled taurine raises the possibility that taurine has some neuro-humoral role in the cerebellar cortex.