Regulation of the frequency‐dependent facilitation of L‐type Ca2+ currents in rat ventricular myocytes.

Abstract

1. An increase in the rate of stimulation induces an augmentation of L-type Ca2+ currents (Ica) and concomitant slowing of current decay in rat ventricular cells. This facilitation is quasi immediate (1-3 s), graded with the rate of stimulation, and occurs only from negative holding potentials. We investigated this effect using trains of stimulation at 1 Hz and the whole-cell patch-clamp technique (18-22°C).2. The decay of Ica is normally bi-exponential and comprises fast and slow current components (Ica fe and Ica,sc respectively). Facilitation of Ica was observed only when ica,fc was predominant.3. Facilitation developed during the run-up of 'Ca with the interconversion of Ica,sc into ica,fC and vanished during the run-down of 'Ca with the loss of 'ca,fc. Ni2" (300 /AM) and nifedipine (1 M) suppressed facilitation owing to the preferential inhibition of Ica,fc-4. Facilitation of Ica was not altered (when present) or favoured (when absent) by the cAMP-dependent phosphorylation of Ca2" channels promoted by isoprenaline or by intracellular application of cAMP or of the catalytic subunit of protein kinase A (C-sub). A similar effect was observed when the dihydropyridine agonist Bay K 8644 was applied.In both cases, facilitation was linked to a preferential increase of 'ca,fe 5. Following intracellular application of inhibitors of protein kinase A in combination with a non-hydrolysable ATP analogue, Ica consisted predominantly of Ica,sc and no facilitation was observed. The calmodulin antagonist naphthalenesulphonamide had no effect on facilitation. 6. When Bay K 8644 was applied in combination with isoprenaline, cAMP or C-sub, the decay of Ica was slowed with the predominant development of Ica,s'c and facilitation of Ica was nearly abolished. Facilitation also depended on extracellular Ca2+, and was suppressed when Ba2" replaced Ca2+ as the permeating ion.7. When no EGTA was included in the patch pipette, facilitation was not further enhanced but a use-dependent decrease of Ica frequently occurred. When BAPTA was used in place of EGTA, the rate of inactivation of Ica was reduced and facilitation was abolished.8. In conclusion, the facilitation of Ica that reflects a voltage-driven interconversion of Ica,fe into Icasc is also regulated by Ca2' and by cAMP-dependent phosphorylation. The presence of the gating pattern typified by 'Ca fe is required. Ca21 may exert its effect near the inner pore of the Ca21 channel protein and control the distribution between the closed states of the two gating pathways.

Voltage-gated calcium channel currents in human coronary myocytes. Regulation by cyclic GMP and nitric oxide.

“…example, the ratio I Ca /I Ba is much higher (Ͼ 0.6) in cardiomyocytes (6,40,41). The cardiac I Ca,L also exhibit much faster decay kinetics (18,(40)(41)(42) with the time constants of inactivation of I Ca,L and I Ba,L differing by Ͼ 10-fold (9).…”

“…The cardiac I Ca,L also exhibit much faster decay kinetics (18,(40)(41)(42) with the time constants of inactivation of I Ca,L and I Ba,L differing by Ͼ 10-fold (9). The presence of a marked Ca 2ϩ -dependent inactivation (6,40), may, at least in part, explain the difference.…”

Mechanisms of L-Type Ca 2+ Current Downregulation in Rat Atrial Myocytes During Heart Failure

“…30 Indeed, as in other species and myocyte types, [31][32][33] the I Ca of rat atrial myocytes is highly sensitive to phosphatase inhibition; these enzymes appear to play an important role in the basal regulation of the current, keeping with the tight coupling between Ca 2ϩ channels and OA-sensitive phosphatases. 34 Various studies have shown that the kinetics of I Ca inactivation also depend on cAMPdependent regulation of the current 35,36 and the slowly decaying I Ca in HF may be another consequence of its downregulation. However, other mechanisms may explain the slow rate of I Ca inactivation in atrial myocytes from HF.…”

Two distinct functional effects of protein phosphatase inhibitors on guinea‐pig cardiac L‐type Ca2+ channels.

“…Besides conventional /8-adrenergic stimulation, for instance, the frequency dependence of Ca21 current (Lee, 1987) could be linked to such a phosphorylation-linked mechanism (Tiaho, Piot, Nargeot & Richard, 1994).…”