On the molecular regulation of muscarinic K+ channels by acetylcholine in cardiac cell membrane

$13 FUNCTIONAL G-PROTEINS.

AND

PHARMACOLOGICAL

HIROYUKI SUGIYAMA*, O k a z a k i 444, Japan.

PROPERTIES

National

OF

Institute

for

GLUTAMATE

RECEPTORS

Physiological

COUPLED

Sciences,

TO

Myodaiji,

When o o c y t e s of A f r i c a n frogs (Xenopus laevis) were i n j e c t e d with mRNA e x t r a c t e d from rat brains, the cells were found to a c q u i r e electrophysiological responsiveness to e x t e r n a l l y applied excitatory amino acids. Analyses of t h e current responses of the c e l l s to e x c i t a t o r y a m i n o a c i d s and t h e i r a n a l o g u e s r e v e a l e d that the i n j e c t i o n of brain mRNA induced at least three d i f f e r e n t s u b t y p e s of e x c i t a t o r y amino acid receportors. The first one was activated by k a i n a t e (KA), and the s e c o n d and t h e t h i r d by b o t h g l u t a m a t e a~d q u i s q u a l a t e (QA). The first and the second showed p r o p e r t i e s characteristic ~o t r a d i t i o n a l l y known KAand Q A - p r e f e r r i n g receptors, respectively, and were thus c o n s i d e r e d to c o r r e s p o n d to them. Functional and p h a r m a c o l o g i c a l p r o p e r t i e s of the third one, h o w e v e r , did not c o r r e s p o n d to those of any of known g l u t a m a t e r e c e p t o r subtypes. Detailed analyses l e d us to c o n c l u d e that this receptor stimulated G-protein-mediated i n o s i t o l p h o s p h o l i p i d m e t a b o l i s m and caused e l e c t r o p h y s i o l o g i c a l responses through an i n t r a c e l l u l a r messenger of i n o s i t o l trisphosphate. Thus, its r e a c t i o n is of "metabotropic" nature. Although this receptor is a c t i v a t e d by QA, o t h e r pharmacological p r o p e r t i e s were c l e a r l y d i f f e r e n t from those of the t r a d i t i o n a l l y known " i o n o t r o p i c " QA receptors. In a d d i t i o n to the glutamate receptors, the m R N A - i n j e c t e d oocytes developed MI muscarinic and $I s e r o t o n e r g i c receptors. Modulatory e f f e c t s of p r o t e i n k i n a s e s on c u r r e n t r e s p o n s e s e v o k e d by t h e s e G - p r o t e i n - c o u p l e d receptors were examined. A c t i v a t i o n of d i f f e r e n t p r o t e i n k i n a s e s s u p p r e s s e d the current r e s p o n s e s meditated by a l l of these G - p r o t e i n - c o u p l e d receptors. Each k i n a s e s u p p r e s s e d the r e s p o n s e s to a s i m i l a r extent i r r e s p e c t i v e l y of the r e c e p t o r s i n v o l v e d , s u g g e s t i n g that, in the oocytes, the e n t i r e s p a n of the r e a c t i o n s e q u e n c e e v o k e d by the m e t a b o t r o p i c g l u t a m a t e r e c e p t o r m a y be c o m m o n to t h a t e v o k e d by MI or $I r e c e p t o r s .

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CELL

YOSHIHISA Hongo,

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