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The negative inotropic effect of aqueous fraction (AqF) obtained from the acetic extract of Psidium guajava L leaf was investigated on the guinea pig left atrium. Myocardial force was measured isometrically (27 ± 0.1 ºC, 2 Hz). AqF (100 &#956;g/ml) reduced contractility of about 85 ± 9.4 % (n = 4, p < 0.001, Fcalc = 51.70, F(0.01; 4; 21) = 5.09, EC50 = 14.28 ± 3 &#956;g/mL) in a concentration-dependent fashion. This effect was reduced by 20 mM of tetraethylammonium (TEA), increasing EC50 to 50 ± 7 &#956;g/ml (n = 4, p < 0.001, Fcalc = 282.13; F(0.01; 21; 66) = 2.36). AqF (100 &#956;g/ml) shifted to the right the CaCl2 concentration-effect curve, increasing the EC50 from 2170 ± 112 to 2690 ± 132 &#956;M (n = 3, p < 0.001, Fcalc = 220.80 ; F(0.01; 29; 60) = 2.19). L-NAME (100 &#956;M) did not modify the AqF inotropic effect (n = 3, p > 0.05) sugesting that the oxide nitric pathway did not participate of the action mechanism of AqF. We can conclude that AqF depresses the atrial contractile by reducing the calcium entry in myocardial cells and also by openenig potassium channels of cardiac tissue. O efeito inotrópico da fração aquosa (AqF) do extrato acético das folhas de Psidium guajava L. foi investigado em átrio esquerdo de cobaia. A força miocárdica foi medida isometricamente (27 ± 0,1 ºC; 2 Hz). A AqF (100 &#956;g/mL) reduziu a contratilidade em até 85 ± 9,4 % (n = 4; p < 0,001; Fcalc = 51,70; F(0,01; 4; 21) = 5,09; CE50 = 14,28 ± 3 &#956;g/mL) de forma dependente da concentração. Este efeito foi reduzido pelo tetraetilamônio (TEA, 20 mM) que também aumentou a CE50 de 14,28 ± 3 &#956;g/mL para 50 ± 7 &#956;g/mL (n = 4; p < 0,001; Fcalc = 282,13; F(0,01; 21; 66) = 2,36). A AqF (100 &#956;g/mL) deslocou para a direita a curva concentração-efeito do CaCl2, aumentando a CE50 de 2170 ± 112 para 2690 ± 132 &#956;M (n = 3; p < 0,001; Fcalc = 220,80 ; F(0,01; 29; 60) = 2,19). Por outro lado, o L-NAME (100 &#956;M) não alterou o efeito inotrópico da AqF (n = 3; p > 0,05), sugerindo que a via do óxido nítrico não participa do mecanismo de ação da AqF. Conclui-se que a AqF deprime a contratilidade atrial por reduzir a entrada de cálcio nas células miocárdicas e por abrir canais de potássio deste tecido

Extract from leaf of Psidium guajava L depresses the guinea pig atrial contractility by interfering with potassium and calcium channels