| pA2 online © Copyright 2004 The British Pharmacological Society |
093P
GKT, University of London Winter Meeting December 2003 |
|
Characterisation
of a canine isolated left atrial preparation to determine inotropic
effects of phosphodiesterase 3 and 4 inhibitors and ß-adrenoceptor
agonists. |
|
Print abstractAlterations in myocardial contractility (inotropy) are major concerns for cardiac safety pharmacology. In-vivo haemodynamic studies in the dog are unable to easily discriminate between direct and in-direct effects. Therefore, our aims were 1) to use the canine isolated left atrial (LA), electrically field stimulated (EFS) preparation to measure the direct effects of the phosphodiesterase (PDE) 3 inhibitor amrinone and PDE4 inhibitors ariflo and rolipram (Shahid & Rodger, 1991) in the presence and absence of the b-adrenoceptor agonist, isoprenaline.Heart tissues, including left atrium, were removed
Heart tissues, including left atrium, were removed from beagle dogs (10-20 kg) euthanised under anaesthesia induced by pentobarbitone (0.5ml/kg) for the use in several pharmacological studies. Upon removal, the tissues were immediately immersed in ice-cold Krebs'. The LA was dissected and 1 x 4 mm strips (up-to 8 strips/dog) were mounted in 15 ml organ baths containing oxygenated Krebs' (37°C, 1 g isometric resting tension) and caused to contract by EFS (10 V, 5 msec, 1 Hz). After a 90 min equilibration period, cumulative concentration response curves (CRC) were constructed to isoprenaline (10-9- 10-4M). Following a 45 min wash out, the curve was re-constructed to determine reproducibility. In a separate set of tissues, following the first curve, CRC were performed to amrinone, ariflo and rolipram (10-8-10-4M). Finally, studies were performed to determine isoprenaline-stimulated responses, following a 15 min pre-incubation with 10-6 or 10-5M PDE inhibitors. All values are expressed as a percentage of the maximum tension developed (Emax) by the first isoprenaline curve. Data are mean EC50± s.e.mean, n-values refer to the number of dogs. Data were compared using Student's t-test, with significance set at P<0.05.
Table 1: Mean EC50 and Emax values (± s.e.mean) for the different treatment groups.
| Treatment |
EC50 µM)
|
Emax
|
n
|
| Isoprenaline curve 1 |
0.68±0.12
|
100
|
18
|
| Isoprenaline curve 2 |
0.73±0.10
|
108±2.8+
|
18
|
| Rolipram (10-8-10-4M) |
No effect
|
No
effect
|
10
|
| Amrinone (10-8-10-4M) |
ND
|
<15%
|
4
|
| Ariflo (10-8-10-4M) |
ND
|
<30%
|
10
|
| Isoprenaline + 10-6 M rolipram |
1.08±0.39
|
129±5.3*
|
4
|
| Isoprenaline + 10-5 M rolipram |
1.06±0.40
|
144±5.2*
|
6
|
| Isoprenaline + 10-6 M amrinone |
0.58±0.18
|
113±7.6
|
4
|
| Isoprenaline + 10-5 M amrinone |
0.29±0.10*
|
136±6.5*
|
4
|
| Isoprenaline + 10-5 M ariflo |
1.15±0.37
|
142±9.2*
|
6
|
+Significantly different from curve 1 (p<0.05). *Significantly different from curve 2 (p<0.05). Isoprenaline + PDE inhibitors were compared to isoprenaline curve 2 to allow for sensitisation, ND=not determined (curve not complete at 10-4M).
In agreement with previous studies, rolipram did not have any direct effect (Katano & Endoh, 1992). Ariflo at 10-4M produced variable increase in tension. The PDE4 inhibitors significantly enhanced the Emax of the isoprenaline curve with no alteration to potency. The PDE3 inhibitor, amrinone had a weak inotropic effect, consistent with previous studies (Shahid & Rodger, 1991). Amrinone at 10-4M enhanced both the potency and efficacy of isoprenaline. These data indicate that both PDE3 and PDE4 inhibitors have weak direct inotropic activity in the dog LA. These weak effects were enhanced by b-adrenoceptor agonism (used to mimic sympathetic stimulation). However, these required high concentrations of the PDE inhibitors and again suggest a minor direct effect.
Katano, Y & Endoh,
M. (1992). J.Cardiovasc.Pharmacol., 20, 715-722.
Shahid, M & Rodger, I (1991). J.Pharm.Pharmacol., 43,
88-94.