Characterization of β-adrenoceptors responsible for venom production in the venom gland of the snake Bothrops jararaca

We have shown that the stimulation of β-adrenoceptors is an important step in venom production in the Bothrops jararaca venom gland. In the present study, the pharmacological profile of the β-adrenoceptor present in Bothrops jararaca venom gland was characterized by radioligand binding assay and by the ability of isoprenaline to promote accumulation of cyclic AMP in dispersed secretory cells. In both cases, the venom glands were obtained from non-extracted snakes (quiescent stage) or from snakes which venom was extracted 4 days before sacrifice (venom production stimulated stage). [ 125I]-iodocyanopindolol ([ 125I]-ICYP) bound to extracted gland membranes in a concentration-dependent and saturable manner, but with low affinity. Propranolol, β 1- or β 2-selective adrenoceptors ligands displaced the [ 125I]-ICYP binding with low affinity, while selective β 3-adrenoceptor ligands did not displace the [ 125I]-ICYP binding. The displacement of [ 125I]-ICYP by propranolol was similar in non-extracted and extracted glands, showing the presence of β-adrenoceptors in both stages. In dispersed secretory cells of non-extracted glands, isoprenaline (1 μM) increased the cyclic AMP production and propranolol (10 μM) was able to block this effect. On the other hand, in extracted glands, isoprenaline had no effect. The results suggest that the β-adrenoceptors present in the Bothrops jararaca venom glands are different from those (β 1, β 2 or β 3) described in mammals, but are coupled to the Gs protein, like the known β-adrenoceptor subtypes. Moreover, previous in vivo stimulation of venom production desensitizes the β-adrenoceptors system and, although the receptors could be detected by binding studies, they are not coupled to the Gs protein, indicating that β-adrenoceptors stimulation contributes to the initial steps of venom synthesis.