Makoto Murakami (1), Naofumi Tokutomi (2,*), Yoshiko Tokutomi (2), Kimio
Tomita (1) and Katsuhide Nishi (2)
Departments of (1) Internal Medicine and (2) Pharmacology, Kumamoto University
School of Medicine, Kumamoto 860-0811, Japan
(*) To whom correspondence should be addressed.
Abstract: We have recently found that mouse megakaryocytes responded
to extracellular alkalinization to pH > 8.0, generating a K+
current under voltage-clamped conditions with the whole cell recording mode
of the patch-clamp technique. The purpose of this study was to physiologically
and pharmacologically characterize the alkaline-dependent K+
conductance of the megakaryocyte membrane. The alkalinization-induced K+
current (IALK) did not seem to be Ca2+-dependent since
IALK was allowed to be generated under intracellularly Ca2+-buffered
conditions with 10 mM EGTA, which completely prevented the generation of
caffeine-induced Ca2+-activated currents of mouse megakaryocytes;
and no [Ca2+]i elevation was evoked by the alkalinization
protocol in contrast to a significant increase in [Ca2+]i
in response to caffeine when [Ca2+]i was measured
with a fura 2 ratiometry. IALK was strongly suppressed with tetraethylammonium
(TEA), 4-aminopyridine (4-AP) and streptomycin (SM), but was completely
resistant to quinidine (QND). The values of IC50 for the suppression
of IALK with TEA, 4-AP and SM were 5.6, 0.47 and 1.5 mM, respectively.
Voltage-gated K+ currents (IK) of the same megakaryocyte
preparation were weakly suppressed with TEA and 4-AP, while they were significantly
suppressed with either SM or QND. These results suggest that mouse megakaryocytes
possess K+ conductance that was activated by extracellular alkalinization
and that probably differs from conventional K+ conductance in
its pharmacological properties.
Keywords: Alkalinization-induced K+ current, Mouse megakaryocyte,
Voltage-gated K+ current, K+ channel blocker, pH-response