Inhibition by Cyclic AMP of Phorbol Ester‐Potentiated Norepinephrine Release from Guinea Pig Brain Cortical Synaptosomes

Abstract

The involvement of Ca2+/phospholipid-dependent protein kinase (protein kinase C, PKC) and cyclic AMP-dependent protein kinase in the K+-evoked release of norepinephrine (NE) was studied using guinea pig brain cortical synaptosomes preloaded with [3H]NE. 12-O-Tetradecanoylphorbol-13-acetate (TPA), a potent activator of PKC, enhanced the K+-evoked release of [3H]NE, in a concentration-dependent manner, but with no effect on the spontaneous outflow and uptake of [3H]NE in the synaptosomes. The apparent affinity of the evoked release for added calcium but not the maximally evoked release was increased by TPA (10(-7) M). Inhibitors of PKC, polymyxin B, and a more potent inhibitor, staurosporine, counteracted the TPA-induced potentiation of the evoked release. Both forskolin and dibutyryl cyclic AMP (DBcAMP) enhanced the evoked release, but reduced the TPA-potentiated NE release. A novel inhibitor of cyclic AMP-dependent protein kinase, KT5720, blocked both the forskolin-induced increase in the evoked release and its inhibition of TPA-induced potentiation in the evoked release, thereby suggesting that forskolin or DBcAMP counteracts the Ca2+-dependent release of NE by activating cyclic AMP-dependent protein kinase. These results suggest that the activation of PKC potentiates the evoked release of NE and that the activation of cyclic AMP-dependent protein kinase acts negatively on the PKC-activated exocytotic neurotransmitter release process in brain synaptosomes of the guinea pig.

Presynaptic Sites of Isolated Canine Saphenous Veins Are More Sensitive to Protein Kinase C than Postsynaptic Ones.

“…PKC activators and inhibitors had no effect on the spontaneous tritium overflow in agree ment with the results in other tissues (10,30,31,35,37), suggesting that PKC may be re sponsible for modulating rather than for mediating NE release from the canine saphe nous veins.…”

“…PDBu also enhanced significantly the KCl-evoked tritium overflow in the presence of phentolamine, indicating that the enhancement caused by PDBu is not due to blockade of presynaptic a 2-adreno ceptors. Furthermore, it has been reported that inhibition of neuronal NE reuptake is not responsible for the phorbol ester-potentiated NE release (30,31). PKC activation causes blockade of K+ currents (23) or facilitation of Na+ (32) and Ca 2+ (33) currents, each of which results in an enhancement of NE re lease.…”

Interleukin-1 Increases Vacuolar-type H+-ATPase Activity in Murine Peritoneal Macrophages

“…Since cAMP and phorbol esters have each been shown to modulate V-ATPase activity (32-34), studies were performed to examine the contribution of these two signaling pathways to the IL-1-induced increase in V-ATPase activity. Two specific inhibitors of cAMP-dependent PKA, H-89 (35) and KT5720 (36,37), markedly abrogated the increase in pH i recovery induced by IL-1 without affecting recovery rates in control cells (Fig. 5, A and B, respectively).…”

Presynaptic plasticity: The regulation of Ca2+-dependent transmitter release

“…One pathway that potentiates presynaptic secretion acts through protein kinase C (PKC, see Tanaka, 1984;Nichols et al,, 1987;Shu and Selmanoff, 1988;Shuntoh et al, 1988; see also Fig. 3).…”