RESUMO
5-hydroxytryptamine-4 (5-HT(4)) receptors have been proposed to contribute to the generation of atrial fibrillation in human atrial myocytes, but it is unclear if these receptors are present in the hearts of small laboratory animals (e.g. rat). In this study, we examined presence and functionality of 5-HT(4) receptors in auricular myocytes of newborn rats and their possible involvement in regulation of gap junctional intercellular communication (GJIC, responsible for the cell-to-cell propagation of the cardiac excitation). Western-blotting assays showed that 5-HT(4) receptors were present and real-time RT-PCR analysis revealed that 5-HT(4b) was the predominant isoform. Serotonin (1 microM) significantly reduced cAMP concentration unless a selective 5-HT(4) inhibitor (GR113808 or ML10375, both 1 microM) was present. Serotonin also reduced the amplitude of L-type calcium currents and influenced the strength of GJIC without modifying the phosphorylation profiles of the different channel-forming proteins or connexins (Cxs), namely Cx40, Cx43 and Cx45. GJIC was markedly increased when serotonin exposure occurred in presence of a 5-HT(4) inhibitor but strongly reduced when 5-HT(2A) and 5-HT(2B) receptors were inhibited, showing that activation of these receptors antagonistically regulated GJIC. The serotoninergic response was completely abolished when 5-HT(4), 5-HT(2A) and 5-HT(2B) were simultaneously inhibited. A 24 h serotonin exposure strongly reduced Cx40 expression whereas Cx45 was less affected and Cx43 still less. In conclusion, this study revealed that 5-HT(4) (mainly 5-HT(4b)), 5-HT(2A) and 5-HT(2B) receptors coexisted in auricular myocytes of newborn rat, that 5-HT(4) activation reduced cAMP concentration, I(Ca)(L) and intercellular coupling whereas 5-HT(2A) or 5-HT(2B) activation conversely enhanced GJIC.
Assuntos
Junções Comunicantes/metabolismo , Átrios do Coração/citologia , Miócitos Cardíacos/metabolismo , Receptor 5-HT2A de Serotonina/metabolismo , Receptor 5-HT2B de Serotonina/metabolismo , Receptor 5-HT2C de Serotonina/metabolismo , Receptores 5-HT4 de Serotonina/metabolismo , Adenilil Ciclases/metabolismo , Aminobenzoatos/farmacologia , Animais , Animais Recém-Nascidos , Western Blotting , Células Cultivadas , Conexinas/metabolismo , Junções Comunicantes/efeitos dos fármacos , Técnicas In Vitro , Indóis/farmacologia , Miócitos Cardíacos/efeitos dos fármacos , Técnicas de Patch-Clamp , Fosforilação/efeitos dos fármacos , Piperidinas/farmacologia , Ratos , Ratos Wistar , Receptor 5-HT2A de Serotonina/genética , Receptor 5-HT2B de Serotonina/genética , Receptor 5-HT2C de Serotonina/genética , Receptores 5-HT4 de Serotonina/genética , Reação em Cadeia da Polimerase Via Transcriptase Reversa , Serotonina/farmacologia , Antagonistas do Receptor 5-HT2 de Serotonina , Antagonistas do Receptor 5-HT4 de Serotonina , Serotoninérgicos/farmacologia , Antagonistas da Serotonina/farmacologia , Sulfonamidas/farmacologia , para-AminobenzoatosRESUMO
The effects of 10 mM caffeine (CAF) on intramembrane charge movements (ICM) were studied in isolated guinea-pig ventricular heart cells with the whole-cell patch-clamp technique. In the presence of CAF, the properties (voltage dependence, maximum Q(ON) [Q(max)], availability with voltage) of Q(ON) charge activated from -110 mV were barely affected. Following a 100 ms prepulse to -50 mV to decrease the participation of charges originating from Na channels, the voltage dependence of Q(ON) was shifted by 5 mV (negative component) and by 10 mV (positive component) towards negative potentials, and Q(max) was depressed by 16.5%. CAF drastically reduced in a time- and voltage-dependent manner Q(OFF) on repolarization to -50 mV, the effects being greater at positive potentials. CAF-induced Q(OFF) immobilization could be almost entirely removed by repolarization to voltages as negative as -170 mV. In these conditions, the voltage-dependence of Q(OFF) (repolarization to +30 to -170 mV) was shifted by 17 mV (negative component) and 30 mV (positive component) towards negative potentials, suggesting an interconversion into charge 2. Most of CAF effects were suppressed when the sarcoplasmic reticulum (SR) was not functional or when the cells were loaded with BAPTA-AM. We conclude that CAF effects on ICM are likely due to Ca(2+) ions released from the SR, and which accumulate in the subsarcolemmal fuzzy spaces in the vicinity of the Ca channels. Because CAF effects were more pronounced on Q(OFF) than on Q(ON) the channels have likely to open before Ca(2+) ions could affect their gating properties. It is speculated that such an effect on gating charges might contribute to the Ca-induced inactivation of the Ca current.
Assuntos
Cafeína/farmacologia , Ativação do Canal Iônico/efeitos dos fármacos , Potenciais da Membrana/efeitos dos fármacos , Miocárdio/metabolismo , Função Ventricular Esquerda/efeitos dos fármacos , Função Ventricular Esquerda/fisiologia , Animais , Canais de Cálcio/metabolismo , Separação Celular , Cobaias , Ativação do Canal Iônico/fisiologia , Potenciais da Membrana/fisiologia , Miocárdio/citologia , Perfusão , Retículo Sarcoplasmático/efeitos dos fármacos , Retículo Sarcoplasmático/metabolismoRESUMO
A comparative study of the effects of formaldehyde and glutaraldehyde on the inward rectification in skeletal muscle fibre of the toad Bufo marinus was made using two different techniques. In whole sartorius muscle fibres formaldehyde produced a transient increase in conductance, followed by a decrease while glutaraldehyde only produced the second effect, with a rather fast time course. In cut end single fibres both aldehydes produced only a reduction of conductance and eventually the abolition of the inward rectification. The blocking effect of glutaraldehyde was concentration and voltage dependent while formaldehyde only produced a concentration dependent blocking. The elimination of inward rectifying currents by aldehydes allowed an estimation of the time constants of activation by a substraction method
