RESUMO
AIMS/BACKGROUND: Diltiazem reduces systemic blood pressure by decreasing the vascular smooth muscle tone. In the liver however, diltiazem seems to cause vasoconstriction, as evidenced by increases in portal pressure. The questions raised by this observation are concerned with a) the site of action of diltiazem (large vessels or sinusoids), b) the formation of permeability barriers and c) the role of Ca2+. The experiments in the present study should provide an answer to these questions. METHODS: The experimental system was the hemoglobin-free perfused rat liver. The multiple-indicator dilution technique was employed with simultaneous injection of [14C]sucrose and [3H]water. Mean transit times and distribution spaces were calculated from the normalized outflow profiles. RESULTS: Calcium alone did not affect the hemodynamics of the liver. Diltiazem, however, changed several hemodynamic parameters when Ca2+ was present, but it was inactive in the absence of this cation. The hemodynamic effects of 500 microM diltiazem were: a) diminution of the transit time through the large vessels (t(o)) and, consequently, of the accessible vascular space (66.9%); b) diminution of the mean transit time of [14C]sucrose (tsuc) and, consequently, of the accessible sinusoidal space (28.1%); c) diminution of the mean transit time of tritiated water (twater) and, consequently, of the accessible cellular space (68.9%); d) diminution of the cellular to extracellular space ratio (theta) from 1.42 +/- 0.05 to 0.46 +/- 0.11. CONCLUSIONS: The linear superposition of the tritiated water and labeled sucrose curves, predicted by Goresky's model, could be optimized even when the curves were obtained with diltiazem + Ca2+, indicating that the distribution of both tracers was still flow-limited. The hemodynamic effects of diltiazem seem to be restricted to a vasoconstriction of the great vessels, an action which was strictly dependent on Ca2+. At the concentration of 500 microM, the effects of diltiazem were pronounced to the point of excluding completely about 2/3 of the liver parenchyma from the microcirculation, as indicated by the observed reduction in the accessible cell space. The sinusoids that were still supplied with perfusion fluid suffered considerable distension (2.19 fold) because the whole perfusate flow was deviated into the remaining 1/3 microcirculatory units. Diltiazem did not seem to induce the formation of intrahepatic shunts or diffusion barriers in the liver.
Assuntos
Anti-Hipertensivos/farmacologia , Cálcio/metabolismo , Diltiazem/farmacologia , Fígado/efeitos dos fármacos , Animais , Hemodinâmica/efeitos dos fármacos , Fígado/fisiologia , Masculino , Perfusão , Ratos , Ratos WistarRESUMO
Diltiazem causes vasoconstriction in the liver when present at high concentrations, an action that is strictly Ca2+-dependent. Diltiazem is also active on energy metabolism. This toxic action could be partly a consequence of hemodynamic effects. In the absence of Ca2+, the hemodynamic effects are no longer present and, consequently, Ca2+-free experiments are useful for distinguishing between hemodynamics-dependent and hemodynamics-independent effects. The experimental system used was the hemoglobin-free perfused rat liver from fed and fasted rats. Diltiazem was infused at various concentrations in the presence and absence of Ca2+. Several metabolic parameters were measured: lactate and pyruvate production (glycolysis), glycogenolysis, oxygen uptake, gluconeogenesis, and the cellular levels of lactate, pyruvate, glucose, AMP, ADP, and ATP. The effects of diltiazem can be divided into three groups: (1) Effects that are strictly dependent on the Ca2+-mediated hemodynamic action. This group comprises inhibition of oxygen uptake at all concentrations (50-500 micromol/L) inhibition of lactate, pyruvate, and glucose release at high concentrations; the decrease in cellular ATP; the increase in cellular AMP; and the cellular accumulation of glucose and lactate. (2) Effects that are independent of the hemodynamic action. The most relevant effect of this type is inhibition of gluconeogenesis. (3) Effects that are influenced by Ca2+ but are independent of the hemodynamic effects. This is the typical case of lactate and glucose release from endogenous glycogen, whose stimulation by low diltiazem concentrations is more pronounced in the presence of Ca2+ than in its absence.
Assuntos
Cálcio/fisiologia , Diltiazem/farmacologia , Metabolismo Energético/efeitos dos fármacos , Hemodinâmica/fisiologia , Fígado/metabolismo , Nucleotídeos de Adenina/metabolismo , Animais , Ingestão de Alimentos , Jejum , Frutose/metabolismo , Gluconeogênese/efeitos dos fármacos , Glicólise/efeitos dos fármacos , Hemodinâmica/efeitos dos fármacos , Técnicas In Vitro , Fígado/efeitos dos fármacos , Glicogênio Hepático/metabolismo , Masculino , Perfusão , Ratos , Ratos WistarRESUMO
The hemodynamic effects of diltiazem in the liver are strictly Ca2+ -dependent Consequently, Ca2+ -free perfusion can be used for investigating the metabolic effects of diltiazem without interference by hemodynamics. Livers were perfused with Krebs/Henseleit-bicarbonate buffer (pH 7.4). For performing Ca2+ -free perfusion the cation was omitted from the perfusion fluid and the cellular pools were exhausted by repeated phenylephrine infusions. Three conditions were investigated with and without Ca2+ : (1) substrate-free perfusion fluid; (2) 0.3 mM [1-(14)C]octanoate infusion; (3) 0.3 mM [1-(14)C]palmitate infusion. The following results were obtained: 1. Oxygen uptake stimulation caused by octanoate and palmitate was abolished by 500 microM diltiazem in the presence of Ca2+; in the absence of Ca2+ there was no inhibition (octanoate) or it was much smaller (palmitate); 2. The 14CO2 production was inhibited in the presence of Ca2+; in the absence of Ca2+ there was no inhibition (palmitate) or even stimulation (octanoate). 3. Ketogenesis from endogenous sources, from palmitate and from octanoate was inhibited by diltiazem in the presence as well as in the absence of Ca2+. The beta-hidroxybutyrate/acetoacetate ratio was diminished in the presence and in the absence of Ca2+ . It was concluded that inhibition of fatty acid oxidation by diltiazem depends partly on the Ca2+ -dependent hemodynamic effects and partly on a Ca2+ -independent action on some enzymatic system.