Interrelationships of ultrastructure and function in the microvasculature of normal and ischaemic myocardium

High-pressure freezing and freeze substitution of rat myocardium for immunogold labeling of connexin 43

The value of high-pressure freezing (HPF) and freeze substitution (FS) for immunoelectron microscopy (immuno-EM) of the heart was investigated in bioptic specimens taken from isolated hearts of 0-, 5-, and 14-day-old rats at baseline and at 15, 30, 45, and 60 min after induction of ischemia. The target antigen chosen here was the gap junction protein connexin 43 (Cx43). After HPF and FS, immunogold labeling was applied for detection of Cx43. Gold particles associated with gap junction areas, free plasma membrane, and annular gap junctions (AGJs) were counted and distributions compared by contingency table analysis. HPF and FS resulted in excellent preservation of antigenicity for Cx43. The mostly good preservation of the ultrastructure was limited by mechanical damage at the border and by ice crystal formation in the center of the tissue blocks. In normal myocardium of newborns, gold particles associated with free plasma membrane were frequently observed, with AGJs only seldom. In older rats, the opposite relation was found. During ischemia, no distribution changes occurred in newborn or 14-day-old rats. In 5-day-old rats, however, ischemia induced a shift of Cx43 from gap junction plaques to AGJs. In conclusion, HPF and FS are an ideal alternative to chemical fixation for immuno-EM as the excellent preservation of antigenicity is combined with a well-preserved ultrastructure. The results indicate that the process of degradation of gap junctions via AGJs gradually increases during postnatal rat heart development, a process that may be accelerated by ischemia in an early developmental state.

Flow heterogeneity following global no-flow ischemia in isolated rabbit heart

The purpose of this study was to evaluate flow heterogeneity and impaired reflow during reperfusion after 60-min global no-flow ischemia in the isolated rabbit heart. Radiolabeled microspheres were used to measure relative flow in small left ventricular (LV) segments in five ischemia + reperfused hearts and in five nonischemic controls. Relative flow heterogeneity was expressed as relative dispersion (RD) and computed as standard deviation/mean. In postischemic vs. preischemic hearts, RD was increased for the whole LV (0.92 +/- 0.41 vs. 0.37 +/- 0.07, P < 0.05) as well as the subendocardium (Endo) and subepicardium considered separately (1.28 +/- 0.74 vs. 0.30 +/- 0.09 and 0.69 +/- 0.22 vs. 0.38 +/- 0.08; P < 0.05 for both comparisons, respectively) during early reperfusion. During late reperfusion, the increased RD for the whole LV and Endo remained significant (0.70 +/- 0.22 vs. 0.37 +/- 0.07 and 1.06 +/- 0.55 vs. 0.30 +/- 0.09; P < 0.05 for both comparisons, respectively). In addition to the increase in postischemic flow heterogeneity, there were some regions demonstrating severely impaired reflow, indicating that regional ischemia can persist despite restoration of normal global flow. Also, the relationship between regional and global flow was altered by the increased postischemic flow heterogeneity, substantially reducing the significance of measured global LV reflow. These observations emphasize the need to quantify regional flow during reperfusion after sustained no-flow ischemia in the isolated rabbit heart.