Effects of Octopamine on Calcium Action Potentials in Insect Oocytes

Our experiments show that octopamine receptors are present on the developing follicles of an insect, Rhodnius prolixus. Application of D,L-octopamine decreased the duration and overshoot of calcium-dependent action potentials (APs), and increased the intrafollicular concentration of cyclic AMP. The threshold concentration of D,L-octopamine for the reduction in electrical excitability was between 1 and 5×10−7moll−1, and maximal effects of a 40–50% reduction in AP overshoot and duration were apparent at 10−4moll−1. At concentrations above 10−5moll−1, a small (<10%) hyperpolarization of the resting potential was also apparent. Effects of D,L-octopamine on oocyte excitability were independent of these small shifts in resting potential. Current injection experiments, in which calcium entry was blocked by cobalt, demonstrated that D,L-octopamine reduced membrane resistance at both hyperpolarizing and depolarizing potentials. Octopamine did not affect the maximum rate of rise of the AP, dV/dtmax, which is an indicator of inward calcium current. It is suggested that octopamine may mediate its effects on excitability through an increase in a voltage-dependent potassium conductance. Application of other phenolamines indicated a rank order of potency of D, Loctopamine > D,L-synephrine > tyramine. The α-adrenergic agonists clonidine, naphazoline and tolazoline were without significant effect at 10−5-10−3moll−1. Reduction of excitability by D,L-octopamine was effectively blocked by phentolamine and metoclopramide. Yohimbine and gramine were less effective as antagonists. Possible functions of octopamine receptors in insect follicles are discussed.