Effects of vanadate on the c-AMP system of the heart

300 μM vanadate (NH4VO3) caused an about 65% increase in force of contraction in isolated electrically driven cat papillary muscles. The positive inotropic effect (PIE) was accompanied by only a small (about 20%) but significant increase in c-AMP levels. 0.3 μM isoprenaline (ISO) produced a much greater increase in c-AMP levels (about 80%) while the PIE of ISO was as great as the PIE observed with 300 μM NH4VO3. In cat right ventricular adenylate cyclase preparations NH4VO3 and ISO stimulated adenylate cyclase activity by nearly the same extent. Propranolol (1 μM and 5 μM) prevented the ISO-induced adenylate cyclase stimulation but not the stimulation due to NH4VO3. Phosphodiesterase activity was not affected by NH4VO3 (up to 1000 μM). It is concluded that the small vanadate-induced increase in c-AMP levels, which is due to adenylate cyclase stimulation, inintact papillary muscles maycontribute to its PIE, but other mechanisms may be at least equally important.