Sources of Ca2+ for contraction of the heart tube of Tenebrio molitor (Coleoptera: Tenebrionidae)
Insect and vertebrate hearts share the ability to generate spontaneously their rhythmic electrical activity, which triggers the fluid-propelling mechanical activity. Although insects have been used as models in studies on the impact of genetic alterations on cardiac function, there is surprisingly little information on the generation of the inotropic activity in their hearts. The main goal of this study was to investigate the sources of Ca2+ for contraction in Tenebrio molitor hearts perfused in situ, in which inotropic activity was assessed by the systolic variation of the cardiac luminal diameter. Increasing the pacing rate from 1.0 to 2.5 Hz depressed contraction amplitude and accelerated relaxation. To avoid inotropic interference of variations in spontaneous rate, which have been shown to occur in insect heart during maneuvers that affect Ca2+ cycling, experiments were performed under electrical pacing at near-physiological rates. Raising the extracellular Ca2+ concentration from 0.5 to 8 mM increased contraction amplitude in a manner sensitive to L-type Ca2+ channel blockade by D600. Inotropic depression was observed after treatment with caffeine or thapsigargin, which impair Ca2+ accumulation by the sarcoplasmic reticulum (SR). D600, but not inhibition of the sarcolemmal Na+/Ca2+ exchanger by KB-R7943, further depressed inotropic activity in thapsigargin-treated hearts. From these results, it is possible to conclude that in T. molitor heart, as in vertebrates: (a) inotropic and lusitropic activities are modulated by the heart rate; and (b) Ca2+ availability for contraction depends on both Ca2+ influx via L-type channels and Ca2+ release from the SR.