Opposite action of β1- and β2-adrenergic receptors on Cav1 L-channel current in rat adrenal chromaffin cells

Abstract

Voltage-gated Ca2+ channels of chromaffin cells are modulated by locally released neurotransmitters through autoreceptor-activated G-proteins. Clear evidence exists in favor of a Ca2+ channel gating inhibition mediated by purinergic, opioidergic, and α-adrenergic autoreceptors. Few and contradictory data suggest also a role of β-adrenergic autoreceptors (β-ARs), the action of which, however, remains obscure. Here, using patch-perforated recordings, we show that rat chromaffin cells respond to the β-AR agonist isoprenaline (ISO) by either upmodulating or downmodulating the amplitude of Ca2+ currents through two distinct modulatory pathways. ISO (1 μM) could cause either fast inhibition (∼25%) or slow potentiation (∼25%), or a combination of the two actions. Both effects were completely prevented by propranolol. Slow potentiation was more evident in cells pretreated with pertussis toxin (PTX) or when β1-ARs were selectively stimulated with ISO + ICI118,551. Potentiation was absent when the β2-AR-selective agonist zinterol (1 μM), the protein kinase A (PKA) inhibitor H89, or nifedipine was applied, suggesting that potentiation is associated with a PKA-mediated phosphorylation of L-channels (∼40% L-current increase) through β1-ARs. The ISO-induced inhibition was fast and reversible, preserved in cell treated with H89, and mimicked by zinterol. The action of zinterol was mostly on L-channels (38% inhibition). Zinterol action preserved the channel activation kinetics, the voltage-dependence of the I-V characteristic, and was removed by PTX, suggesting that β2AR-mediated channel inhibition was mainly voltage independent and coupled to Gi/Go-proteins. Sequential application of zinterol and ISO mimicked the dual action (inhibition/potentiation) of ISO alone. The two kinetically and pharmacologically distinct β-ARs signaling uncover alternative pathways, which may serve the autocrine control of Ca2+-dependent exocytosis and other related functions of rat chromaffin cells.