Ca(2+)-activated K+ channel is present in guinea-pig but lacking in rat hepatocytes.
The mechanisms of norepinephrine-induced membrane responses in isolated hepatocytes from guinea-pigs and rats were compared using the suction-pipette, patch-clamp method, and intracellular Ca2+ concentration ([Ca2+]i) was measured using the Ca2+ fluorescent dye, Quin 2. The resting membrane potentials of isolated guinea-pig hepatocytes were -50 +/- 1 mV (mean +/- SD; n = 38), which is similar to that previously reported in rat hepatocytes by Sawanobori et al. (J Cell Physiol 139: 580-585, 1989). In guinea-pig hepatocytes, norepinephrine (6 microM) caused a membrane hyperpolarization, and norepinephrine (6 microM) or Ca(2+)-ionophore (A23187) (0.4 microM) caused a corresponding outward current. The sensitive current produced by norepinephrine and Ca(2+)-ionophore reversed its polarity at -74 +/- 9 mV (n = 7). The single channel recorded by cell-attached patch and inside-out patch had mean conductance of around 20 + 1 pS and was activated by 1 microM [Ca2+]i. On the other hand, neither norepinephrine (6-20 microM) nor Ca(2+)-ionophore (A 23187) (0.4 microM) caused any change in membrane potential and current in rat hepatocytes, whereas norepinephrine increased [Ca2+]i both in rat and guinea-pig hepatocytes to a similar degree. In the single-channel recording, we recorded single channels that had a mean conductance of 109.8 +/- 17.7 pS different from around 20 pS in guinea-pig. In inside-out patches, increased Ca2+ concentration from 10(-6) to 10(-3) M at the intracellular face of the membrane did not modify the single channel of rat hepatocytes. These results indicate that increased [Ca2+]i activates this channel in guinea-pigs, but that the channel activated by increased [Ca2+]i is lacking in rat hepatocytes membrane. Therefore, different mechanism operates in different species of liver cells to keep the constant state.
Takanashi
H
H
Third Department of Internal Medicine, Tokyo Medical and Dental University, Japan.
Sawanobori
T
T
Kamisaka
K
K
Maezawa
H
H
Hiraoka
M
M
D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism; D004594:Electrophysiology; D006168:Guinea Pigs; D066298:In Vitro Techniques; D017136:Ion Transport / Q000187:drug effects; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D009638:Norepinephrine / Q000494:pharmacology; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D013045:Species Specificity
D015221:Potassium Channels; D000001:Calcimycin; D002118:Calcium; D009638:Norepinephrine