O uso da microcalorimetria de titulação isotérmica na avaliação do metabolismo celular de ilhotas de langerhans de ratos wistar

Abstract

Calorimetry consists of the measure of the heat (potency, q, in W) released or absorbed at constant temperature as a function of the time (t) (isothermal calorimetry) or as a function of the variation of the temperature (T) (scanning calorimetry) in a given biological phenomenon, and microcalorimetry is simply the measure of this heat in the micro-watt scale (microJ/s). The biological microcalorimetry, being a no specific and no destructive technique, allows the sample or system to be analyzed be a solution of biochemical compounds or a suspension of cells or microorganisms. Due to the shortage of data using Isothermal Titration Calorimetry (ITC) to evaluate the cellular metabolism, allied to the previous experience in the use of Heat Conduction Microcalorimetry (HCM) in the studies of the metabolism of promastigotes of Leishmania (L.) amazonnensis, in the present work ITC was used to evaluate the functional state of two different cellular types: promastigotes of Leishmania (Leishmania) amazonensis and rat Langerhans islets. Using ITC, the flow of metabolic heat liberated by promastigotes of Leishmania incubated with fructose in the concentration of 2.5 mmol.L-1 was evaluated as 204 pW, having been previously evaluated by HCM as being 105 pW, in the same experimental conditions. It was suggested that the largest sensibility of ITC and the biological variation of the samples can explain the difference found among the values of heat released using the two types of microcalorimeters. This result allowed us to validate the use of ITC for evaluation of the cellular metabolism of other cellular types. Seeking to use this technique (ITC) in the evaluation of the viability of the islets of Langerhans for transplants, it was intended to evaluate microcalorimetricaly the effects of the glucose on mice Langerhans islets. In those experiments it was evident a significant increase of the metabolic heat produced by the islets samples submitted to two consecutive concentrations of 2.8 and 16.3 mmol.L-1 of glucose. Additionally, it was determined the amount of secreted insulin and lactate concentration in the islets samples incubated in these glucose concentrations; the ratios of the values (for high/low glucose) of secreted insulin, released heat and concentration of lactate were significantly larger than the unit. The index (or ratio) of lactate showed a value (2.91 ± 0.50) that can suggest a smaller aerobic use of glucose and a larger use of glycolisys when the islets were submitted to the high glucose concentration in the tested conditions. For the secreted insulin the ratio was 1.67 ± 0.30, in spite of the global averages values for high and low glucose concentrations are statistically similar for this parameter. The value of the ratio for released heat (1.72 ± 0.13) was similar to the value of the ratio for secreted insulin, and that raises the important possibility of the index (or ratio) of the released heat to substitute the index (or ratio) of secreted insulin in the evaluation of the viability of the islets of Langerhans for transplants, with several possible practical advantages in relation to the methods used currently. In short, by those studies it was possible to demonstrate the viability of the use of Isothermal Titration Calorimetry (ITC) for the evaluation of the cellular metabolism of promastigotes of Leishmania (L.) amazonensis and of rat Langerhans islets; this technique can be adapted for the study of the metabolic viability of other cellular types.