Qualitative differentiation of positive inotropic effects of isoprenaline and 8-bromo-cAMP in isolated rat atria

58 CHANGES IN ACCESSIBILITY OF MEMBRANE PROTEOLIPIDS TO SURFACE LABELLING INDUCED BY MODIFICATION OF MEMBRANE VISCOSITY IN CARDIAC CELL CULTURES. M. Mersel, A. Amar, A. Benenson, M. Heller, P. Padieu and A. Pinson. Hebrew University-Hadassah Medical School, Jerusalem and Laboratoire de Biochimie Medicale, Dijon. The Lactoperoxidase-catalyzed radioiodination of the lipid moieties of the sarcolemma of heart cells in culture was used to study the labelling of proteolipid components as a function of membrane fluidity. Membrane fluidity (measured by EPR) was modified by adding either erucic acid (increased fluidity) or stearic acid (decreased fluidity) to the culture medium. By comparison to controls, more 125 I labelling was associated with surface proteolipids of cells grown in presence of stearic acid (less fluid) and less radioactivity was associated with the proteolipids of erucic acid-incubated cultures (more fluid). This indicates that the proteolipid moieties of the sarcolemma were less accessible to enzymatic radioiodination when the cell membrane fluidity increased. It is postulated that increase of sarcolemmal fluidity causes the surface proteolipid to “sink” rendering it less accessible for labellin since proteolipids are involved in the proper functioning of membrane Na+ + K $ ATPase and Ca +2ATPase. These findings might explain previous observations showing that erucic acid diminished the force of contraction of cardiac muscle and depressed overshoot potential level in cultured heart cells.

QUALITATIVE DIFFERENTIATION OF POSITIVE INOTROPIC EFFECTS OF ISOPRENALINE AND 8-BROMO-CAMP IN ISOLATED RAT ATRIA. Timo Metsa-Ketela. Department of Biomedical Sciences,University of Tamper-e, Tampere, Finland. Isoprenaline produces a biphasic response on the parameters indicating myocardial relaxation (Metsa-Ketela, Adv. in Myocardiology,vol.3, in press). The relaxation appears only secondly and the initial response mimics that of hypercalcemia. In the present study, the effects of isoprenaline (O.luM) and 8-bromo-CAMP (2mM) on contraction-relaxation cycle (CRC) of spontaneously beating rat atria were compared. The results are listed in the following table: Isoprenaline 8-bromo-CAMP 4’ 15” 60” 0 1’ 100 101.3+0.81 145.8:S.b 118.1:3.1* 206.2+13.8* Amplitude T*jmax / T’ ‘min 1.0 0.960+0.0? 0.981+0.016 1.015+0.022*0.887+0.026* Duration of CRC 100 97.4+0.87*103.4+2.7 102.4zl.2’ 89.9-+2.2* *pdO.O5 compared to O-value. T”= second derivative of CRC, The results indicate that the responses are qualitatively different. This may imply that the mechanisms of inotropic esfects of isoprenaline and 8-bromo-CAMP are not identical. Grant: The Orion and Medica Scientific ‘Foundation, Finland.