Developmental Changes in β-Adrenergic Modulation of L-Type Ca 2+ Channels in Embryonic Mouse Heart

Abstract In the adult mammalian myocardium, cellular Ca 2+ entry is regulated by the sympathetic nervous system. L-type Ca 2+ channel currents are markedly increased by β-adrenergic (β-A) agonists, which contribute to changes in pacing and contractile activity of the heart. In the developing mammalian heart, the regulation of Ca 2+ entry by this enzyme cascade has not been clearly established, because changes in receptor density and coupling to downstream elements of the signaling cascade are known to occur during embryogenesis. In this study, we systematically investigated the regulation of L-type Ca 2+ channel currents during development of the murine embryonic heart. We used conventional whole-cell and perforated-patch–clamp procedures to study modulation of L-type Ca 2+ channel currents and to assay functional activity of distinct steps in the β-A signaling cascade in murine embryonic myocytes at different stages of gestation. Our data indicate that L-type Ca 2+ channels in early-stage (day-11 to -13) myocytes are unresponsive to either isoproterenol or cAMP. L-type Ca 2+ channels in late-stage (day-17 to -19) murine myocytes, however, exhibit responses to isoproterenol and cAMP similar to responses in adult cells, providing evidence that the β-A cascade becomes functionally active during this period of embryonic development. We found that L-type Ca 2+ channel activity in early-stage cells is increased by cell dialysis with the catalytic subunit of cAMP-dependent protein kinase (cA-PK) and that dialysis of early-stage cells with the holoenzyme of cA-PK restores functional responses to forskolin and cAMP, but not to isoproterenol. Our results provide strong evidence that a key factor in the early-stage insensitivity of L-type Ca 2+ channels to cAMP is the absence, or low expression level, of the holoenzyme of cA-PK but that in addition, another element in the signaling cascade upstream from adenylate cyclase is expressed at a nonfunctional level or is uncoupled from the cascade and thus contributes to L-type Ca 2+ channel insensitivity to β-A agonists in early stages of the developing murine heart.