Histamine, released from mast cells in atheromatous plaque, has been known to cause cardiac ischemia or sudden cardiac death in atherosclerosis patient. Previous reports have suggested that histamine induced coronary vasoconstriction was due to increase in IP(3) and DAG, which induce release of Ca²⁺ from SR and increase the Ca²⁺ sensitivity of contractile element via activation of PKC. Recently, it was reported that application of histamine cause depolarization of intestinal smooth muscle, which may contribute to histamine-induced contraction via augmenting Ca²⁺ influx through activation of Ca²⁺ channels. However, the underyling mechanism of histamine-induced depolarization and its contribution to the magnitude of coronary vasoconstriction are still uncertain.

To elucidate the underlying mechanism of Ca2+ influx change during histamine-induced vasoconstriction, we examined the effect of Ca²⁺ channel antagonist and PKC blocker on histamine-induced contractions, and then measured the effect of PKC antagonist on whole cell K⁺ current using patch clamping method in rabbit coronary smooth muscle cells.

Application of histamine induced phasic and tonic constraction of coronary rings via activation of H(1) receptors. Pretreatment of Ca²⁺ channel antagonist (nifedipine, 1 µM) or PKC blockers (10 nM staurosporine and 10 µM Gö6976) markedly inhibited histamine-induced tonic contraction, which suggest that the magnitude of tonic contraction depend on the Ca²⁺ influx. Application of 4-AP, a blocker of voltage-dependent K⁺ channels, increased resting tone of coronary rings, and combined treatment of nifedipine blocked this 4-AP induced increase of resting tone. Application of active analoge of DAG (1,2-DiC₈) significantly inhibited the activity of voltage-dependent K⁺ current in single smooth muscle cell, meanwhile the inactive analogue of DAG (1,3-DiC₈) has no apparent effect on the activity of voltage-dependent K⁺ current. Furthermore, pretreatment of calphostin C (1 µM), a blocker of PKC, diminished the 1,2-DiC₈-induced inhibition of K⁺ current.

PKC dependent inhibition of voltage-dependent K⁺ current may be responsible for the maintaining of histamine-induced tonic contraction in rabbit coronary artery.