An unusual Ca2+ entry pathway activated by protein kinase C in dog splenicartery

The characteristics of the Ca2+ entry pathways that are activated by protei n kinase C (PKC) in canine splenic artery were investigated. Phorbol 12, 13 -dibutyrate (PDB) contracted tissues and increased Ca2+ influx. PDB-induced contraction was reduced by preincubation of tissues in Ca2+-free Krebs' so lution (1 mM EGTA) but was unaffected when Ca2+-free solution was applied a fter contraction was initiated with PDB. In contrast, Ca-45 influx and cont raction induced by PDB were resistant to nifedipine, Cd2+, Gd3+, La3+, or N i2+ whether added before or during exposure to PDB. Indeed, Cd2+ reduced Ca -45(2+) efflux and potentiated Ca2+ influx, but not PDB-induced contraction . Norepinephrine (NE)-induced contractions were inhibited by preincubation in Ca2+-free Krebs' solution (1 mM EGTA). Nifedipine (10 mu M) led to a sma ll reduction in the NE-induced contraction but was without effect on Ca-45( 2+) influx. Pretreatment for 16 min with Cd2+, Gd3+, or La3+ (each 1 mM) re duced or abolished NE-induced contraction and Ca2+ influx. Application of t hese cations after exposure to NE did not affect Ca-45(2+) influx but reduc ed tension. The Q(10) for the increase in Ca-45(2+) influx was approximatel y 2 for high K+ and NE, but 4 for PDB. The results suggest that stimulation of PKC in dog splenic artery activates a Ca2+ entry pathway that is resist ant to di- and trivalent cations. The inhibition of Ca2+ influx by preincub ating with cations during short-term exposure to NE may represent an action on Ca2+ turnover that precedes activation of PKC.