Phorbol 12,13-dibutyrate increases vascular tone but has a dual action on intracellular calcium levels in porcine coronary arteries

The effects of phorbol 12,13-dibutyrate (PDBu), a protein kinase C activator, on the tone and intracellular calcium level (Ca_infi^sup2+) of vascular smooth muscles were simultaneously measured by use of a force-displacement transducer and the fura-2 microscopic fluorometric technique in porcine coronary arteries. Cumulatively applied PDBu produced a slowly developing and concentration-dependent contraction in the concentration range of 10^-8-10^-6 mol/l. Contractions induced by PDBu were sustained after removal of PDBu. Changes in Ca_infi^sup2+produced by PDBu were very slight, although Ca_infi^sup2+was increased at lower concentrations (3 × 10^-7 and 10^-7 mol/l) and decreased at higher concentrations (3 × 10^-7 and 10^-7 mol/l). Verapamil 10^-7 mol/l, partially inhibited the contractions throughout the concentration range of PDBu and the increase in Ca_infi^sup2+induced by lower concentrations of PDBu. When the effects of PDBu were compared with those of the 90 mmol/l KCl medium, the force of contraction induced by a single concentration of 10^-7 mol/l, PDBu was about 50% and the increase in Ca_infi^sup2+was about 10%. Removal of extracellular calcium by 1 mmol/l EGTA decreased Ca_infi^sup2+by about 20% but vascular tone did not change. PDBu (10^-7 mol/l) produced a small contraction without an increase in Ca_infi^sup2+in the Ca²⁺-free medium. Depolarization by the 20 mmol/l KCl medium increased Ca_infi^sup2+by about 20%, whereas vascular tone did not change. The contraction and increase in Ca_infi^sup2+induced by 10^-7 mol/l, PDBu were both enhanced in the 20 mmol/l KCl medium. The contraction induced by 10^-7 mol/l, PDBu was more than 100%, whereas Ca_infi^sup2+decreased by 10-20%. Contractions induced by 10^-7 mol/I PDBu were partially inhibited in the Ca_infi^sup2+-free medium and were not potentiated in the 20 mmol/l KCl medium. These results suggest that activation of protein kinase C by phorbol ester has a dual action on Ca_infi^sup2+: a slight increase and a slight decrease. The contractions induced by phorbol ester not only depend on an increase in Ca_infi^sup2+but involve the sensitization of contractile system to calcium or the generation of force via a Ca²⁺-independent process.