Effects of muscarinic toxins MT2 and MT7, from green mamba venom, on m1, m3 and m5 muscarinic receptors expressed in Chinese Hamster Ovary cells.

Several small proteins called muscarinic toxins (MTs) have been isolated from venom of green mamba (Dendroaspis angusticeps). They have previously been shown in radioligand binding studies to have high selectivity and affinity for individual muscarinic receptor subtypes, but less is known of their functional effects. This study has examined the actions of two of these MTs, MT2 and MT7, using changes in cytosolic Ca(2+) ([Ca(2+)](i)) measured using the fluorescent indicator fura-2 in Chinese Hamster Ovary (CHO) cells stably transfected with individual muscarinic receptor subtypes, m1, m3 and m5. MT2 activated the m1 receptor: at concentrations above 100 nM it caused significant and concentration-dependent increases in [Ca(2+)](i). From 25 to 800 nM MT2 also produced increases in [Ca(2+)](i) by activating m3 receptors, although these increases in [Ca(2+)](i) were not strictly concentration-dependent with only intermittent responses being recorded (i.e. it was not always possible to obtain a response to the agonist with each application of the compound). MT2 (800-1600 nM) also caused significant increases in [Ca(2+)](i) in CHO cells expressing the m5 muscarinic receptor subtype. MT7 (1 microM) displayed no agonist activity at any of the muscarinic receptors but was a potent non-competitive antagonist (at 20 nM) at the m1 muscarinic receptor subtype. It had no antagonist activity at the m3 or m5 subtypes. These results indicate that MT7 is a highly specific antagonist at the m1 muscarinic receptor subtype as suggested by results from radioligand binding studies. However, MT2 is less selective for the m1 muscarinic receptor than previously described as it also exhibits agonist activity at the m3 and m5 muscarinic receptors, which was not detected in radioligand binding studies.