Norepinephrine pretreatment attenuates Ca2+ overloading in rat trabeculae during subsequent metabolic inhibition: improved contractile recovery via an alpha 1-adrenergic, PKC-dependent signaling mechanism

The present study was designed in order to investigate more precisely the role of calcium homeostasis maintenance in protein kinase C (PKC) mediated preconditioning. We used a 15 min pre-incubation period, with 1 mumol/l exogenous norepinephrine (NE) to pharmacologically precondition isolated, superfused rat trabeculae against contractile dysfunctioning following 120 min of metabolic inhibition (MI, in 2 mmol/l CN- containing Tyrode without glucose at 1 Hz stimulation frequency). Contractile recovery was studied during a subsequent 60 min recovery period (RP, in glucose containing Tyrode at 0.2 Hz). Tyrode was gassed with 95%, O2/ 5% CO2 and kept at a constant temperature of 24 degrees C. Force and intracellular free calcium ([Ca2+]i) were monitored throughout the experimental protocol; [Ca2+]i was measured using fura-2. Pretreatment with NE (group NE-I) significantly increased the fraction of trabeculae that resumed to contract during RP, from 36 +/- 13% (mean +/- S.E.M.) in controls to 82 +/- 10% (P < 0.05). In correspondence with this, NE-pretreatment increased the proportion of trabeculae in which the Ca2+ rise from the onset of rigor development during MI was attenuated. After 40 min of MI [Ca2+]i in the failing control, as well as failing group NE-I, trabeculae (1.08 +/- 0.20 and 1.51 +/- 0.26 mumol/l, respectively) was increased significantly compared to the mean value registered in the recovering preparations of these groups (0.34 +/- 0.04 mumol/l: P < 0.05). Specific inhibition of PKC with 2 mumol/l chelerythrine (group NE-IV) almost completely blocked the protection induced by NE-pretreatment, including its protective action against Ca2+ overload, i.e. the fraction of trabeculae that resumed to contract during RP returned to untreated control level (46 +/- 11%: P < 0.05 v group NE-I). Also in this case [Ca2+]i in the failing group NE-IV trabeculae after 40 min of MI was increased substantially, compared to the value measured in the recovering preparations (4.75 +/- 1.00 and 0.60 +/- 0.08 mumol/ l, respectively). The relative importance of both alpha-adrenergic and beta-adrenergic receptor pathways in this preconditioning-like effect of NE-pretreatment, was investigated using specific blockers. The results point to an alpha 1-adrenergic receptor mediated signaling mechanism, which enhances PKC-dependent control of [Ca2+]i from the onset of rigor development during MI.