Nitric oxide synthase activity and endothelial ultrastructure in ischaemic skin-flaps

The aim of the present study was to detect and quantify nitric oxide synthase (NOS) activity and to investigate morphological changes in the endothelium in two different ischaemic dorsal flap models in the rat, one based cranially and one based caudally. Intact skin from the dorsum was used as control. In both groups flaps were removed at 1, 4, 12, 24 and 72 h after surgery respectively. NOS-activity was measured by the conversion of L-arginine to L-citrulline and endothelial morphology was investigated using transmission electron microscopy. Intact skin showed Ca²⁺-dependent but no Ca²⁺-independent NOS-activity. A time-dependent decrease in Ca²⁺-dependent NOS-activity was seen in the proximal and distal part of the flaps in both flap models and was most pronounced in the distal part. Ca²⁺-independent NOS-activity increased in the proximal and distal part of flaps based cranially and in the proximal part of flaps based caudally. Morphological analysis of the endothelium showed signs of endothelial damage including blebbed membranes, swelling and endothelial loss. These findings show that ischaemia caused by skin-flap surgery leads to endothelial damage and a decrease in Ca²⁺-dependent nitric oxide synthase activity. Furthermore, in the skin-flaps an induction of Ca²⁺-independent nitric oxide synthase (iNOS) activity was noted both in surviving flap tissue and in flap tissue destined to necrose.