RESUMO
Osmolarity reduction (20%) elicited 3H-norepinephrine (NE) efflux from rat cortical synaptosomes. The hyposmotic NE release resulted from the following events: (i) a Na+-dependent and La3+-, Gd3+- and ruthenium red-sensitive depolarization; (ii) a cytosolic Ca2+ ([Ca2+]i) rise with contributions from external Ca2+ influx and internal Ca2+ release, probably through the mitochondrial Na+-Ca2+ exchanger; and (iii) activation of a [Ca2+]i-evoked, tetanus toxin (TeTX)-sensitive, PKC-modulated NE efflux mechanism. This sequence was established from results showing a drop in the hyposmotic [Ca2+]i rise by preventing depolarization with La3+, and by the inhibitory effects of Ca2+-free medium (EGTA; 50%), CGP37157 (the mitochondrial Na+-Ca2+ exchanger blocker; 48%), EGTA + CGP37157 or by EGTA-AM (> 95% in both cases). In close correspondence with these effects, NE efflux was 92% decreased by Na+ omission, 75% by La3+, 47% by EGTA, 50% by CGP37157, 90% by EGTA + CGP37157 and 88% by EGTA-AM. PKC influenced the intracellular Ca2+ release and, mainly through this action, modulated NE efflux. TeTX suppressed NE efflux. The K+-stimulated NE release, studied in parallel, was unaffected by Na+ omission, or by La3+, Gd3+ or ruthenium red. It was fully dependent on external Ca2+, insensitive to CGP37157 and abolished by TeTX. These results suggest that the hyposmotic events, although different from the K+-evoked depolarization and [Ca2+]i rise mechanisms, are able to trigger a depolarization-dependent, Ca2+-dependent and TeTX-sensitive mechanism for neurotransmitter release.
Assuntos
Cálcio/metabolismo , Córtex Cerebral/citologia , Exocitose/fisiologia , Norepinefrina/metabolismo , Sinaptossomos/fisiologia , Animais , Bário/farmacologia , Cádmio/farmacologia , Bloqueadores dos Canais de Cálcio/farmacologia , Carbazóis/farmacologia , Clonazepam/análogos & derivados , Clonazepam/farmacologia , Interações Medicamentosas , Ácido Egtázico/análogos & derivados , Ácido Egtázico/farmacologia , Inibidores Enzimáticos/farmacologia , Exocitose/efeitos dos fármacos , Indóis/farmacologia , Lantânio/farmacologia , Concentração Osmolar , Ésteres de Forbol/farmacologia , Cloreto de Potássio/farmacologia , Proteína Quinase C/farmacologia , Ratos , Ratos Wistar , Rutênio Vermelho/farmacologia , Sódio/farmacologia , Trocador de Sódio e Cálcio/farmacologia , Espectrometria de Fluorescência/métodos , Sinaptossomos/efeitos dos fármacos , Toxina Tetânica/farmacologia , Tiazepinas/farmacologia , Fatores de Tempo , Trítio/metabolismo , ômega-Conotoxinas/farmacologiaRESUMO
This study was designed to gain additional insight into the mechanism of the slow force response (SFR) to stretch of cardiac muscle. SFR and changes in intracellular Na(+) concentration ([Na(+)](i)) were assessed in cat papillary muscles stretched from 92% to approximately 98% of L(max). The SFR was 120+/-0.6% (n=5) of the rapid initial phase and coincided with an increase in [Na(+)](i). The SFR was markedly depressed by Na(+)-H(+) exchanger inhibition, AT(1) receptor blockade, nonselective endothelin-receptor blockade and selective ET(A)-receptor blockade, extracellular Na(+) removal, and inhibition of the reverse mode of the Na(+)-Ca(2+) exchange by KB-R7943. KB-R7943 prevented the SFR but not the increase in [Na(+)](i). Inhibition of endothelin-converting enzyme activity by phosphoramidon suppressed both the SFR and the increase in [Na(+)](i). The SFR and the increase in [Na(+)](i) after stretch were both present in muscles with their endothelium (vascular and endocardial) made functionally inactive by Triton X-100. In these muscles, phosphoramidon also suppressed the SFR and the increase in [Na(+)](i). The data provide evidence that the last step of the autocrine-paracrine mechanism leading to the SFR to stretch is Ca(2+) entry through the reverse mode of Na(+)-Ca(2+) exchange.