Reciprocal regulation of Ca2+-activated outward K+ channels of Pyrus pyrifolia pollen by heme and carbon monoxide

The regulation of plant potassium (K+) channels has been extensively studied in various systems. However, the mechanism of their regulation in the pollen tube is unclear. In this study, the effects of heme and carbon monoxide (CO) on the outward K+ (K-out(+)) channel in pear (Pyrus pyrifolia) pollen tube protoplasts were characterized using a patch-clamp technique. Heme (1 mu M) decreased the probability of K-out(+) channel opening without affecting the unitary conductance, but this inhibition disappeared when heme was co-applied with 10 mu M intracellular free Ca2+. Conversely, exposure to heme in the presence of NADPH increased channel activity. However, with tin protoporphyrin IX treatment, which inhibits hemeoxygenase activity, the inhibition of the K-out(+) channel by heme occurred even in the presence of NADPH. CO, a product of heme catabolism by hemeoxygenase, activates the K-out(+) channel in pollen tube protoplasts in a dose-dependent manner. The current induced by CO was inhibited by the K+ channel inhibitor tetraethylammonium. These data indicate a role of heme and CO in reciprocal regulation of the K-out(+) channel in pear pollen tubes.