Contribution of different Ca-activated K channels to the first phase of the response to TRH in clonal rat anterior pituitary cells.
OBJECTIVE
Thyrotropin-releasing hormone (TRH) induces biphasic changes in electrical activity, cytosolic free Ca(2+) level ([Ca(2+)](i)), and prolactin secretion from both clonal GH cells and native lactotrophs. The first phase of the TRH response is characterized by hyperpolarization because of activation of Ca(2+)-activated K(+) channels (K(Ca)). In the present study, the relative contribution of BK, SK, and IK channels to the first phase of the TRH response in GH(4) cells was assessed.


METHODS
The expression of IK channels was confirmed by PCR with specific primers for SK4 (IK). The response to TRH was studied using the perforated patch technique and Ca(2+) microfluoromety (fura-2). The involvement of different K(Ca) channels was estimated by employing the specific channel blockers iberiotoxin (BK), apamin (SK) and clotrimazole (IK).


RESULTS
Application of 100 nM iberiotoxin, 1 microM apamin, and 10 microM clotrimazole reduced the peak value of the outward K(+) current during the first phase of the TRH response by 33, 26, and 33%, respectively. Clotrimazole also shortened the duration of the outward current response by 60%, causing a reduction of total charge movement by 73%. All these toxin-induced reductions were significant (P < 0.05). A combination of all three toxins abolished the current response almost completely.


CONCLUSIONS
All the three main types of K(Ca) channels are involved in the first phase of the TRH response, with IK as the major contributor. This is the first demonstration of a dominant role of IK compared with BK and SK channels in excitable cells.
Mørk
H K
HK
Department of Molecular Biosciences, University of Oslo, Oslo, Norway.
Haug
T M
TM
Sand
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D000818:Animals; D001030:Apamin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D045744:Cell Line, Tumor; D002999:Clone Cells / Q000378:metabolism; D003022:Clotrimazole / Q000494:pharmacology; D004594:Electrophysiology / Q000379:methods; D008564:Membrane Potentials / Q000502:physiology; D010455:Peptides / Q000494:pharmacology; D010903:Pituitary Gland, Anterior / Q000166:cytology* / Q000187:drug effects; D026902:Potassium Channel Blockers / Q000494:pharmacology*; D015221:Potassium Channels / Q000378:metabolism*; D012333:RNA, Messenger / Q000032:analysis; D051381:Rats; D013973:Thyrotropin-Releasing Hormone / Q000502:physiology*
D010455:Peptides; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D012333:RNA, Messenger; D001030:Apamin; D013973:Thyrotropin-Releasing Hormone; C064719:iberiotoxin; D003022:Clotrimazole; D002118:Calcium