Cold-Induced Contraction of Newborn Lamb Cerebral Arteries: Role of Na⁺,K⁺-ATPase and Ca²⁺-ATPase in the Potentiation by Sodium Orthovanadate † 358

Infants and children subjected to various degrees of hypothermia during cardiac surgery often express neurological pathology possibly associated with dysfunctional cerebral circulation. In isolated middle cerebral artery (MCA) rings from newborn lambs, hypothermia stimulates marked contraction via a mechanism thought to involve protein tyrosine phosphorylation-dependent signal transduction pathways. Sodium orthovanadate (SOV) an inhibitor of protein tyrosine phosphatases, potentiates cold-induced contraction in MCA, supporting this hypothesis. However, SOV may act via inhibition of Na⁺,K⁺-ATPase and/or Ca²⁺-ATPase. To assess the role of Na⁺,K⁺-ATPase and Ca²⁺-ATPase in the MCA contractile response to hypothermia, we evaluated the effect of their respective inhibitors, ouabain (OB) and thapsigargin (TG) on cold-induced contractile behavior. Isolated MCA ring segments (2-3 mm) from newborn lambs were studied in tissue bath. Continuous bath cooling from 37°C to 17°C(-0.5°C/min)stimulated a temperature-dependent contraction(0.96±22g at 17°C). Pretreatment with OB (10^-4M) partially attenuated the contraction (0.48±0.09 g) and TG pretreatment(10^-5 M) nearly abolished it (0.18±0.06 g). SOV pretreatment(50μM) potentiated the contractile response to cold even in the presence of OB or TG, effectively reversing their inhibitory effect (0.97±0.17 g and 1.03±0.27 g, respectively). In conclusion: 1) SOV potentiated cold-induced contraction in the presence and absence of Na⁺,K⁺-ATPase inhibition and Ca²⁺-ATPase inhibition, 2) both Na⁺,K⁺-ATPase inhibition and Ca²⁺-ATPase inhibition attenuated the cold-induced contraction, an effect opposite to that of SOV. The results suggest that cold-induced contraction requires intact TG-sensitive intracellular Ca²⁺ stores and functional Na⁺,K⁺-ATPase. Nevertheless, the mechanism by which SOV potentiates cold-induced contraction is attributed to inhibition of protein tyrosine phosphatases rather than to inhibition of Na⁺,K⁺-ATPase or Ca²⁺-ATPase.