Distinct guanine nucleotide binding protein alpha-subunit receptor coupling in GH cell lines: effects of bromocriptine and hormones on effector enzyme modulation.
The purpose of the present study was to elucidate how the dopamine agonist bromocriptine affected receptor-effector systems in GH cells by measuring adenylate cyclase (AC) and phospholipase C (PL-C) modulation in cell membrane preparations. To perturb the interaction between the receptor and G-protein, polyclonal antibodies reacting with the predicted C-terminal amino acid sequence of G-protein alpha-subunits were used. The effect of bromocriptine on secretagogue elicited prolactin (PRL) secretion from whole cells was also monitored. Bromocriptine inhibited the basal secretion of PRL in a dose dependent manner, and completely abolished both the thyroliberin (TRH) and the vasoactive intestinal peptide (VIP) stimulated PRL secretion in GH(3) cells. Maximal inhibitory effect on PRL egress elicited by both hormones was obtained at 10-50 microM of bromocriptine. Messenger RNAs for both the short and long form of the D(2) receptor (D(2)R) were demonstrated in all three GH cell lines using the RT-PCR technique, advocating that D(2)Rs are coupled to distinct G-proteins and, thus, probably being responsible for the observed effects of bromocriptine in these cell lines. Basal AC activity, as measured in membrane preparations of GH(3) cells, remained unaffected by bromocriptine treatment (10 microM), while TRH and VIP stimulated AC activities (175% and 350% of control values, respectively) were partially inhibited (by some 50%). This inhibitory effect of bromocriptine was completely and specifically abolished in the presence of an antiserum against G(i2)alpha. Basal PL-C activity was also unaffected by bromocriptine, while TRH stimulated PL-C activity (350% of control value) was inhibited by bromocriptine (10 microM) by approximately 50%. Immunoblocking of G(q/11)alpha, however, reduced the stimulatory effect of TRH on PL-C activation by some 65%, while an antiserum against G(o)alpha partly counteracted the inhibitory effect of bromocriptine (10 microM) on TRH stimulated PL-C activity. Thus, TRH dependent AC stimulation was counteracted by bromocriptine via G(i2). TRH activation of PL-C occurs via G(q/11), while inhibition by bromocriptine appears to involve G(o). These mechanisms probably account for the major part of the actions of bromocriptine, however, other not yet recognised intermediates may be involved.
Johansen
P W
PW
Department of Clinical Pharmacology, The National Hospital, University of Oslo, Oslo, Norway. per.w.johansen@rikshospitalet.no
Paulssen
R H
RH
Bjøro
T
T
Gautvik
K M
KM
Gordeladze
J O
JO
D000262:Adenylyl Cyclases / Q000187:drug effects / Q000378:metabolism; D000818:Animals; D001971:Bromocriptine / Q000494:pharmacology*; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D018491:Dopamine Agonists / Q000494:pharmacology; D020962:Heterotrimeric GTP-Binding Proteins / Q000276:immunology / Q000378:metabolism*; D007106:Immune Sera / Q000276:immunology / Q000494:pharmacology; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000187:drug effects / Q000378:metabolism; D010902:Pituitary Gland / Q000166:cytology; D011388:Prolactin / Q000187:drug effects / Q000378:metabolism; D051381:Rats; D017448:Receptors, Dopamine D2 / Q000819:agonists / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology*; D010738:Type C Phospholipases / Q000187:drug effects / Q000378:metabolism; D014660:Vasoactive Intestinal Peptide / Q000494:pharmacology*
D018491:Dopamine Agonists; D007106:Immune Sera; D017448:Receptors, Dopamine D2; D014660:Vasoactive Intestinal Peptide; D001971:Bromocriptine; D013973:Thyrotropin-Releasing Hormone; D011388:Prolactin; D017853:Phosphotransferases (Alcohol Group Acceptor); C051038:Inositol 1,4,5-trisphosphate 3-kinase; D010738:Type C Phospholipases; D020962:Heterotrimeric GTP-Binding Proteins; D000262:Adenylyl Cyclases