96 Neuroscience Letters, 98 (1989) 96 100 Elsevier ScientificPublishers Ireland Ltd. NSL 05927 Adenosine effects upon the spontaneous quantal transmitter release at the frog neuromuscular junction in the presence of protein kinase C-blocking and -activating agents Dimitrie D. BrS.ni~teanu, Dumitru D.D. BrS.ni~teanu, Adrian Covic, Eugen Brailoiu, Dragomir N. Serban and Ion D. Haulica Department o/Physiology, Institute of Medicine and Pharmacy, la#i (Romania) (Received 20 September 1988; Revised version received 8 November 1988;Accepted 9 November 1988) Key words." Adenosine: Protein kinase C: Quantal release; Neuromuscular junction; Adenosine deami- nase This paper gives experimental evidence involving protein kinase C (PKC) in the inhibitory effects of adenosine (ADO) upon the spontaneous transmitter release at the frog neuromuscular junction. In the presence of two PKC inhibitors -- polymyxin B (5 x 10 6 tool/l) and H-7 (10 -s mol/l), both adenosine (5 × l 0 5tool/l) and its stable analogue I-PIA (5 × 10-s mol/l), significantlyincreased the rate of the spon- taneous release of acetylcholinequanta. Even when PKC was activated with OAG (5 x 10 6mol/1)or TPA (162 x 10 9 tool/l) and quantal release was increased greatly, ADO still inhibited release. ADO deaminase increased the PKC-induced activation of the transmitter release significantly. The inhibitory action of adenosine (ADO) and its derivatives upon neurotransmit- ter release from presynaptic nerve terminals has been widely demonstrated and ac- cepted [8, 17]. Nevertheless, the mechanism(s) of this action are still controversial [19]. Experimental evidence involves a number of possibilities: (1) a reduction in the availability of free internal Ca 2+ [3, 19, 21] which could be the consequence of a reduced presynaptic Ca 2+ influx [1 1, 14-16, 21] or an increased rate of Ca 2+ removal [19]: (2) a adenosine receptor-mediated change of the intraneuronal cAMP concen- tration [4, 8, 9]; and (3) an ADO-mediated increase in K + flux [5]. Recently, it was suggested that ADO could inhibit transmitter release in the neuromuscular junction by decreasing the calcium affinity of certain component(s) of the secretory apparatus [20] through a phosphorylation process involving either a cyclic AMP-dependent protein kinase [4, 20], or a guanine nucleotide binding regulatory subunit [7, 20]. A connection between ADO effects and protein kinase C (PKC) was recently suggested ('m'respomtence: D.D. Brfini~teanu,Dept. Physiology, Institute of Medicine and Pharmacy, 16 Universit- ~'qiiStr.. Ia~i, R-6600, Romania. 0304-3940/89/$ 03.50 ',ff 1989 Elsevier ScientificPublishers Ireland Ltd.