Nerve-evoked constriction of rat tail veins is potentiated and venous diameter is reduced after chronic spinal cord transection.

Despite reduced sympathetic activity below the level of a spinal cord injury (SCI), venoconstriction during autonomic dysreflexia increases venous return to the heart. Here, contractions of isometrically mounted tail veins from rats with spinal transection at T4 performed 8 - 10 weeks earlier are compared with those from sham-operated rats. After SCI, lumen diameter was reduced by ∼30% and the contractions evoked by electrical stimulation of the perivascular axons were larger than control. This augmentation of neurovascular transmission was not associated with enhanced sensitivity to α-adrenoceptor agonists or to adenosine-5'-triphosphate (ATP) although contractions to depolarization with K(+) were larger after SCI. The percentage reduction in nerve-evoked contraction after SCI produced by the α(1)-adrenoceptor antagonist prazosin (10 nM) was unchanged but that by the α(2)-adrenoceptor antagonist rauwolscine (0.1 μM) was reduced. The relative contribution of P2-purinoceptors to nerve-evoked contractions after α-adrenoceptor blockade, revealed by adding suramin (0.1 mM), was unchanged. The greater depolarization-induced contraction and the reduced contribution of α(2)-adrenoceptors to nerve-evoked contraction suggest that changes in the venous smooth muscle underlie the potentiation of neurovascular transmission after SCI. Furthermore, the smaller lumen diameter after SCI will increase the pressure that the veins exert on the luminal contents when they are neurally activated.