The aim of the present study was to assess the inotropic effects of vasoactive intestinal peptide (VIP) on isolated myocardial trabeculae from the right atrium and the left ventricle of human hearts. Furthermore, using reverse transcriptase-PCR, we wanted to determine the presence of mRNAs encoding the three cloned human VIP receptors, VPAC 1 , VPAC 2 and PAC 1 . The trabeculae were paced at 1.0Hz in tissue baths, and changes in isometric contractile force upon exposure to agonist were studied. VIP had a potent positive inotropic effect in some of the atrial and ventricular trabeculae tested. This effect was almost completely blocked by the VIP-receptor antagonist VIP-(6-28). mRNAs encoding the human VPAC 1 , VPAC 2 and PAC 1 receptors were detected in human myocardial trabeculae from both the right atrium and the left ventricle. In conclusion, VIP has a direct positive inotropic effect in both the atria and the ventricles of the human heart. The presence of mRNAs for the VPAC 1 , VPAC 2 and PAC 1 receptors suggest that VIP may mediate its effect via these receptors.

We have investigated the effects of the calcium-sensitizing inotropic agent EMD 57033 on Ca 2+ handling in intact and skinned rat ventricular myocytes. Intracellular Ca 2+ was monitored using fura 2. Myocytes were saponin-skinned, allowing study of sarcoplasmic reticulum (SR) function. In intact myocytes EMD 57033 (1–10 µ mol/l) produced a concentration-dependent decrease in the amplitude of the Ca 2+ transient and prolonged its declining phase, but had no effect on the rise time. In skinned myocytes, the amplitude of spontaneous Ca 2+ release from the SR was decreased by EMD 57033 (5 and 10 µ mol/l), although this agent had no significant effect on the frequency of spontaneous Ca 2+ release. In the presence of the cross-bridge inhibitor 2,3-butanedione monoxime (5 mmol/l), or in a low bathing Ca 2+ concentration (1 mmol/l), EMD 57033 (10 µ mol/l) had smaller effects on both the amplitude and time course of the Ca 2+ transient in intact cells than in the absence of 2,3-butanedione monoxime or in the presence of 2 and 5 mmol/l Ca 2+ respectively. These data suggest that the effects of EMD 57033 on Ca 2+ are due to changes in Ca 2+ binding to troponin C, secondary to cross-bridge formation. Thus, during positive inotropy, EMD 57033 is unlikely to provoke arrhythmias due to effects on SR Ca 2+ handling. In intact cells, its effects on Ca 2+ handling would be expected to protect against arrhythmias.

The aim of the present study was to determine possible inotropic effects mediated by endothelin ET A and ET B receptors in human myocardial trabeculae from the right atrium and the left ventricle. Isolated trabeculae from human hearts were paced at 1.0 Hz in tissue baths, and changes in isometric contractile force upon exposure to agonist were studied. Endothelin-1 (ET-1) and ET-3 had a strong positive inotropic effect in all trabeculae. ET-1 was significantly more potent than ET-3 in both atrial ( P < 0.001) and ventricular ( P < 0.05) trabeculae. Preincubation with the ET A receptor antagonist FR139317 (1 μ M) decreased significantly ( P < 0.005) the potency of ET-1 in both atrial and ventricular trabeculae, without any significant changes in E max (maximum effect obtained with an agonist). The ET B receptor agonist IRL 1620 had a positive inotropic effect only in some trabeculae, and the ET B receptor antagonist BQ 788 (1 μ M) almost completely blocked this effect. These results suggest that both ET A and ET B receptors mediate positive inotropic responses at both the atrial and ventricular level in the human heart.

1. In this study we have investigated the effects of a novel inotropic agent, pimobendan (UDCG 115-BS), on skinned and intact ventricular muscle from ferrets. 2. Pimobendan (20 or 100 μmol/l) increased tension at a given free [Ca 2+ ] when applied to skinned ventricular muscle, i.e. it increased the Ca 2+ sensitivity of the myofibrils. 3. Tension and intracellular free Ca 2+ ([Ca 2+ ] i ) were measured simultaneously in intact papillary muscles using the aequorin technique. When 25 μmol/l pimobendan was added to the superfusing solution, a slowly developing positive ionotropic effect was produced, which was accompanied by an increase in the size of the systolic rise in [Ca 2+ ] i (Ca 2+ transients) with a similar time course. 4. In order to determine whether pimobendan increased the Ca 2+ sensitivity of myofibrils in an intact papillary muscle, we compared the increase in Ca 2+ transients and tension observed in response to changes in extracellular [Ca 2+ ] with those observed in response to pimobendan. The result of this comparison was that in intact muscle pimobendan caused no apparent increase in myofibrillar Ca 2 + sensitivity. 5. Pimobendan caused an abbreviation of the time course of the Ca 2+ transients, but the twitch was slightly prolonged. 6. When isoprenaline was added to the superfusing solution, a positive inotropic effect was produced, which was accompanied by a marked increase in the size of the Ca 2+ transients. Isoprenaline caused an abbreviation of the time course of both the Ca 2+ transients and the twitch. When the Ca 2+ sensitivity of the intact myofibrils was determined as described above, isoprenaline caused a desensitization. Pimobendan produced a sensitization when compared with isoprenaline. 7. These results are consistent with the hypothesis that pimobendan produces an inotropic effect in isolated cardiac muscle which is mediated both by an increase in Ca 2+ sensitivity and by an increase in adenosine 3′: 5′-cyclic monophosphate due to its phosphodiesterase-inhibiting activity. Such a combination of activities may be particularly advantageous for an inotropic agent.