J. Jurevicius et R. Fischmeister, CAMP COMPARTMENTATION IS RESPONSIBLE FOR A LOCAL ACTIVATION OF CARDIAC CA2-ADRENERGIC AGONISTS( CHANNELS BY BETA), Proceedings of the National Academy of Sciences of the United Statesof America, 93(1), 1996, pp. 295-299
The role of cAMP subcellular compartmentation in the progress of beta-
adrenergic stimulation of cardiac L-type calcium current (I-Ca) was in
vestigated by using a method based on the use of whole-cell patch-clam
p recording and a double capillary for extracellular microperfusion, F
rog ventricular cells were sealed at both ends to two patch-clamp pipe
ttes and positioned approximately halfway between the mouths of two ca
pillaries that were separated by a 5-mu m thin wall. I-Ca could be inh
ibited in one half or the other by omitting Ca2+ from one solution or
the other, Exposing half of the cell to a saturating concentration of
isoprenaline (ISO, 1 mu M) produced a nonmaximal increase in I-Ca (347
+/- 70%; n = 4) since a subsequent application of ISO to the other pa
rt induced an additional effect of nearly similar amplitude to reach a
673 +/- 130% increase. However, half-cell exposure to forskolin (FSK,
30 mu M) induced a maximal stimulation of I-Ca (561 +/- 55%; n = 4),
This effect was not the result of adenylyl cyclase activation due to F
SK diffusion in the nonexposed part of the cell. To determine the dist
ant effects of ISO and FSK on I-Ca, the drugs were applied in a zero-C
a solution. Adding Ca2+ to the drug-containing solutions allowed us to
record the local effect of the drugs, Dose-response curves for the lo
cal and distant effects of ISO and FSK on I-Ca were used as an index o
f cAMP concentration changes near the sarcolemma. We found that ISO in
duced a 40-fold, but FSK induced only a 4-fold, higher cAMP concentrat
ion close to the Ca2+ channels, in the part of the cell exposed to the
drugs, than it did in the rest of the cell, cAMP compartmentation was
greatly reduced after inhibition of phosphodiesterase activity with 3
-isobulyl-methylxanthine, suggesting the colocalization of enzymes inv
olved in the cAMP cascade. We conclude that beta-adrenergic receptors
are functionally coupled to nearby Ca2+ channels via local elevations
of cAMP.