The determination of the myocardial extracellular space in the cat in vivo: a comparative methodological study.

The myocardial extracellular space (ECS) was determined in cats in vivo by means of the ECS indicators, inulin and sulfanilic acid and, additionally, as glucose space. Experiments were carried out in cats subjected to bilateral kidney ligation (single injection) and in intact cats (single injection or infusion of indicator). The heart was clamped by instant deep-freezing in situ; this technique was compared, in renally-ligated cats, with excision and subsequent freezing of the heart in liquid nitrogen. The myocardial blood content was significantly decreased, and the myocardial lactate concentration significantly increased in excised, as compared with in situ-clamped hearts. Renally-ligated cats showed marked hypotension. The mean myocardial blood content was also significantly lower than in intact animals. A highly significant correlation was found between myocardial blood content and blood pressure for all experiments with instant deep-freezing. The mean inulin ECS value in renally-ligated cats was 22.7 +/- 1.5 ml. In accordance with the fact that the ECS is dependent on the tissure blood content, the corresponding values in intact animals were significantly higher, 25.9 +/- 2.5 ml (single injection) and 26.3 +/- 3.9 ml (infusion), calculated per 100 g tissue wet weight. Similar values were obtained for the glucose and sulfanilic acid ECS. If the interstitial space, an expression independent of the tissue blood content is used as space parameter, no significant differences were found under any of the present experimental conditions.