Tunnel capillaries in hypertrophied myocardium of rats with aorto-caval fistula

Male inbred rats (Wag) with aorto-caval fistula were used as a model of volume-overloaded heart hypertrophy. Hearts were taken after 1, 2, and 6 months postoperatively for immunohistochemical, histological and ultrastructural investigations. Immunohistochemical staining with some anti-extracellular matrix antibodies and ultrastructural findings allowed us to recognize intracardiocytic tunnels which were intracellular invaginations of plasmalemma surrounded by basement membrane. Inside the tunnels endothelial cells forming well-developed capillaries were entrapped. Tunnel capillary formation is discussed as an adaptive response to increased cardiac work due to volume overload.