Regulation of Ca2+-Activated K+ Channel: Changes with Development

In ovine middle cerebral arteries, it has been shown that the [Ca2+]i change in response to iberiotoxin, a selective BK channel blocker, is significantly different between adult and fetal VSMCs. Our study of basilar VSMCs in whole-cell preparations showed that the total outward current density of fetal myocytes (57.9 ± 6.7 pA/pF) was greater than that of the adult (37.9 ±1.8 pA/pF). This increase in outward current density is contributed by an increase in BK channel current density in fetal myocytes. Excised, inside-out preparations showed that the unitary conductance, as well as the BK channel expression, did not differ between the two age groups. However, voltage-activation curves showed that the BK channel open probability (Po) at the same [Ca2+] was much greater in fetal myocytes than that of the adult. This increased Po in fetal VSMCs was shown to be due to decreased calcium set-point (Ca0). The Ca0 of BK channels were 8.8 and 4.7 μM in adult and fetal myocytes, respectively, suggesting that at the same [Ca2+] and voltage, more fetal BK channels are activated. Thus the increased BK channel current density in fetal myocytes appears to result from a lower Ca0. Several factors, including channel splice variants, micro-environment of the channel, and post-translational modification have been shown to modulate BK channel activity. To follow-up on the study, the underlying mechanisms that affect BK channel activity were examined. The amount of channel-associated protein kinases and phosphatases was examined. Our results indicated that adult myocytes have greater channel-associated PKA activity, while those of the fetus have greater channel-associated PKG activity. Furthermore, the protein phosphatase activity in the fetal myocytes was much greater than that of the adult. Together, the results suggest that BK channels are modulated by protein kinases and phosphatases-phosphorylation/dephosphorylation can change BK channel activity. The hypothesis that Ca0 is modulated by phosphorylation was further tested. This study showed that phosphorylation and dephosphorylation do not change voltage or Ca2+ sensitivity of the channel. Rather, phosphorylation and dephosphorylation change BK channel activity and Ca0 This result suggests that Ca0 may be used as an indication of the extent of BK channel phosphorylation