Opposite Action of β1- and β2-Adrenergic Receptors on Ca_V1 L-Channel Current in Rat Adrenal Chromaffin Cells

Abstract

Voltage-gated Ca(2+) channels of chromaffin cells are modulated by locally released neurotransmitters through autoreceptor-activated G-proteins. Clear evidence exists in favor of a Ca(2+) channel gating inhibition mediated by purinergic, opioidergic, and alpha-adrenergic autoreceptors. Few and contradictory data suggest also a role of beta-adrenergic autoreceptors (beta-ARs), the action of which, however, remains obscure. Here, using patch-perforated recordings, we show that rat chromaffin cells respond to the beta-AR agonist isoprenaline (ISO) by either upmodulating or downmodulating the amplitude of Ca(2+) currents through two distinct modulatory pathways. ISO (1 microm) could cause either fast inhibition (approximately 25%) or slow potentiation (approximately 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 beta(1)-ARs were selectively stimulated with ISO + ICI118,551. Potentiation was absent when the beta(2)-AR-selective agonist zinterol (1 microm), the protein kinase A (PKA) inhibitor H89, or nifedipine was applied, suggesting that potentiation is associated with a PKA-mediated phosphorylation of L-channels (approximately 40% L-current increase) through beta(1)-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 beta(2)AR-mediated channel inhibition was mainly voltage independent and coupled to G(i)/G(o)-proteins. Sequential application of zinterol and ISO mimicked the dual action (inhibition/potentiation) of ISO alone. The two kinetically and pharmacologically distinct beta-ARs signaling uncover alternative pathways, which may serve the autocrine control of Ca(2+)-dependent exocytosis and other related functions of rat chromaffin cells.

Fast exocytosis mediated by T- and L-type channels in chromaffin cells: distinct voltage-dependence but similar Ca2+-dependence

“…Second, L-type channels are shown to eVectively control catecholamine release in in-vivo micro-dialyzed adrenal glands of anesthetized animals, while P/Q and N-type channels contribute marginally (Akiyama et al 2004). Third, L-type channels can be up-and down-modulated by secreted products through the activation of -adrenergic autoreceptors (Hernandez-Guijo et al 1999;Cesetti et al 2003) and play a regulatory function in the depolarization-evoked exocytosis in chromaYn cells Nagy et al 2004). Fourth, recent data suggest an additional role for L-type channels, which can act as Ca 2+ sensor proteins in neuroendocrine secretion (Lerner et al 2006).…”

“…1e). L-type currents have half-maximal activation around ¡8 mV (Cesetti et al 2003) and T-type currents have V ½ at ¡26 mV.…”

“…e I-V curves for T and L-type channels obtained using ramp commands from ¡60 to +60 mV (see Cesetti et al 2003 andGiancippoli et al 2006 for details). Notice the presence of a single peak for the recording of L-type current (grey trace) and the dual peak when measuring the T-type current (black trace).…”

“…In the latter case, the peak at more positive voltage is due to contamination of R-type currents insensitive to -toxins and nifedipine. Adapted from Carabelli et al (2003), Giancippoli et al (2006) and from unpublished data taken from the work of Cesetti et al (2003) 123 increases ( C 1 and C 2 ) and are requested to carry the same quantity of Ca 2+ charge. In this way, from the amplitude of C 2 one can estimate the percentage of vesicle depletion that occurs during the Wrst pulse ( C 1 ) and with simple calculation one can determine the size of IRP and the probability of release (P).…”

“…In addition, it is interesting to assay how close the secretory events follow the voltage-dependence of Ttype channels and how T-type channels may coexist with the other voltage-gated Ca 2+ channels normally expressed in rodent chromaYn cells, in particular with the L-type channels which are the dominant Ca 2+ channel subtypes controlling secretion in these cells (see Garcia et al 2006). Besides being the most densely expressed Ca 2+ channels in rodent chromaYn cells, L-type channels possess other important features: (1) control 30% of catecholamine release in in-vivo perfused rat adrenal glands while P/Q and N-type channels contribute marginally (Akiyama et al 2004), (2) can be up-and down-modulated by -adrenergic autoreceptors (Hernandez-Guijo et al 1999;Cesetti et al 2003) and aVected by phosphodiesterase type-4 activity , (3) contribute to a lowthreshold "pace-maker" current that sets the action potentials autorythmicity at rest (Carbone et al 2006c).…”