[Mechanism of action of sympathetic and parasympathetic neuromediators at the level of the heart cell: use of photosensitive molecules].
We have shown in the first part of this work, how pharmacological technics can be associated to electrophysiological measurements, to study the action of neurotransmitters on the excitation-contraction coupling in cardiac cells. However, to better understand their mechanism of action, it is necessary to have informations about the link between the activation of the receptor and the conductance changes. Kinetics measurements have been very useful to investigate this problem at the nicotinic cholinergic receptor. This study was performed with technics such as iontophoresis, voltage relaxations, noise analysis and recently patch clamp measurements. Nicotinic responses are very fast (in the order of the msec) and are however the best known. The binding of the agonist to the receptor would lead to the opening of a channel only by molecular conformation change. By comparison, beta-adrenergic and muscarinic responses in heart or smooth muscle can be considered as "slow responses" because they are thought to be mediated by intracellular biochemical steps. We have developed a new technique using photosensitive compounds, which allows to jump the extra- or intracellular concentration of some active molecules. The experiments have been performed on frog atrial fibers, in current or voltage clamp conditions using a double sucrose gap technique. Various photochemical reactions have been exploited. The photoisomerization of an azobenzene derivative (Bis-Q) allows to study the muscarinic-induced potassium conductance. Bis-Q binds to muscarinic receptors and blocks the effect of agonists. These drugs have the additional property that their cis and trans configuration have different potencies. The photoisomerization cis----trans or trans----cis is obtained a few msec after a 1 msec duration light flash (respectively visible or UV), and then induces a concentration jump of antagonist near the receptor. The results show that the potassium conductance change has a slow relaxation time constant (tau = 500 msec.) and this suggest rather successive intracellular steps than the opening of a channel directly linked to the receptor. Cyclic nucleotides are considered as the intracellular messengers for various neurotransmitters or hormones. Our photochemical technique allows to jump the intracellular concentration of cAMP and cGMP in a few msec. cAMP (or cGMP) orthonitrobenzyl derivatives are able to penetrate cell membrane without injection and have no effect up to 100 microM. A UV flash (1 msec duration) induces a photolysis of the derivative and generates instantaneously cAMP (or cGMP) inside the cell.(ABSTRACT TRUNCATED AT 400 WORDS)
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D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D002395:Catecholamines / Q000378:metabolism; D000242:Cyclic AMP / Q000378:metabolism; D006152:Cyclic GMP / Q000378:metabolism; D004594:Electrophysiology; D006321:Heart / Q000502:physiology*; D006801:Humans; D007700:Kinetics; D009206:Myocardium / Q000166:cytology; D018377:Neurotransmitter Agents / Q000502:physiology*; D009543:Nifedipine / Q000494:pharmacology; D010275:Parasympathetic Nervous System / Q000502:physiology*; D010777:Photochemistry; D011188:Potassium / Q000494:pharmacology; D011976:Receptors, Muscarinic / Q000187:drug effects; D013564:Sympathetic Nervous System / Q000502:physiology*
D002395:Catecholamines; D018377:Neurotransmitter Agents; D011976:Receptors, Muscarinic; D000242:Cyclic AMP; D006152:Cyclic GMP; D009543:Nifedipine; D000109:Acetylcholine; D011188:Potassium
Mécanisme d'action des neuromédiateurs sympathique et parasympathique au niveau de la cellule cardiaque: utilisation de molécules photosensibles.