CA2+ REGULATION OF HEART CONTRACTILITY IN OCTOPUS

Isometric force development of electrically paced preparations isolate d from the systemic heart of Octopus vulgaris is were utilized to exam ine the regulation of contractility by Ca2+. Increases in extracellula r Ca2+, to the physiological level, resulted in enhancement of twitch force. For instance, at 36 beats.min(-1) an increase Ca2+ from 3 to 9 mmol.l(-1) resulted in a threefold increase in twitch force developmen t, When steady-state contraction at 12 beats.min(-1) was followed by a rest period of either 5 or 10 min, the first contraction always exhib ited either an increase in twitch force or stayed unchanged such that post-rest twitch force was about 133% of the last value in the steady- state train. Ryanodine (12.5 mu mol.l(-1)), which is considered to be a specific inhibitor of the Ca2+ storage and release capabilities of t he sarcoplasmic reticulum (SR)I was applied to further assess Ca2+ han dling. Twitch force fell to about 22% of the preteatment level in prep arations paced at either 12 or 36 beats min(-1). In all preparations t he frequency transition from 12 to 36 beats.min(-1) was associated wit h an increase in resting tension. The increase was 37 +/- 14% prior to ryanodine treatment and was significantly elevated to 127 +/- 33% fol lowing treatment, When steady-state contraction at 36 beats.min(-1) wa s followed by a rest period of 10 s, the first contraction was not sig nificantly different from the last beat in the train prior to ryanodin e; however, with ryanodine treatment, post-rest twitch force developme nt significantly decreased. Twitch force development was regular at pa cing rates of up to 300 beats.min(-1). Twitch force was maintained up to rates of 84 beats.min(-1) but decreased thereafter and reached a va lue of about 10% at 300 beats.min(-1). Resting tension increased subst antially as frequency was elevated from 12 to 36 beats.min(-1) and the n gradually increased as frequency was further elevated to 180 beats.m in(-1). In conclusion, the Octopus ventricle is dependent upon extrace llular Ca2+ for contraction, A post-rest potentiation of force develop ment, the negative impact of ryanodine, and the ability to respond reg ularly at high pacing rates imply a strong reliance on the SR in Ca2cycling based on criteria established for vertebrate hearts.