The L-type Ca2+ current [I(Ca(L))] increases with time after patch
rupture in guinea pig ventricular myocytes dialyzed with pipette solutions containing > or =20 mM
1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid ([
BAPTA]pip). I(Ca(L)) progressively increases because
BAPTA chelates subsarcolemmal Ca2+ to disinhibit cardiac
adenylyl cyclase (AC) activity. We studied inhibition by
carbachol (CCh) of I(Ca(L)) (22-24 degrees C). At 40 mM [
BAPTA]pip, 100 microM CCh reversibly suppressed I(Ca(L)) maximally by 42%; half-maximal inhibition (20%) required 1 microM.
Atropine antagonized the CCh effect on
BAPTA-stimulated I(Ca(L),) as did dialysis with 50 microM guanosine-5'-O-(3-thio)triphosphate. At 20, 30, and 40 mM [
BAPTA]pip, I(Ca(L)) increased by 6.7 +/- 1.8, 10.1 +/- 1.4, and 11.3 +/- 1.2 pA/pF, respectively. Inhibition by 100 microM CCh averaged -1.8 +/- 0.6, -2.3 +/- 0.4, and -4.1 +/- 0.4 pA/pF at 20, 30, and 40 mM [
BAPTA](pip), respectively. Dialysis of the AC inhibitor 2'-dAMP (100 microM) suppressed I(Ca(L)) run up in 40 mM
BAPTA and its inhibition by CCh. Replacing 1.8 mM external Ca2+ with Ba2+, which lacks high-affinity regulatory sites on AC, suppressed CCh-induced inhibition. Neither I(Ca(L)) run up nor its inhibition by CCh occurred when 40 mM
EGTA, a slower
chelator, replaced
BAPTA. Our results support the AC disinhibition hypothesis for
BAPTA. We propose that CCh inhibits I(Ca(L)) in
BAPTA by increasing either AC sensitivity to inhibition by ambient Ca2+ or the activity of the inhibitory
guanine nucleotide binding protein.