097P University of Oxford
BPS 75th Anniversary Meeting December 2006

 

Fade of inotropic responses but not of sinoatrial tachycardia elicited by 5-hydroxytryptamine in atria from new-born piglets. role of phosphodiesterases 3 and 4

1María Luisa Vargas, 1Alejandro Galindo-Tovar and 2Alberto J Kaumann. 1Department of Pharmacology and H.U.V.Arrixaca, Murcia, Spain and 2Department of Physiology, Development and Neuroscience, University of Cambridge, Cambridge CB2 3EG, UK.

 

5-Hydroxytryptamine (5-HT) increases porcine atrial contractility through 5-HT4 receptors but the inotropic responses fade. Fade is prevented by the non-selective phosphodiesterase (PDE) inhibitor isobutyl-methyl-xanthine (De Maeyer et al., 2006) but the role of PDE isoenzymes is unknown (Kaumann & Levy, 2006). We investigated the influence of the PDE3-selective blocker cilostamide (Cil, 300 nM) and PDE4-selective blocker rolipram (Rol, 1μM) on the kinetics of 5-HT responses in spontaneously beating right atria and paced left atria from new-born piglets euthanised with pentobarbital 100mg.kg-1. Experiments were carried out in the presence of (-)-propranolol (200 nM). Cil and Rol increased sinoatrial rate from 76.5±6.2 to 86.0±7.8 bpm (n=14, P<0.02, paired Student´s test) and from 88.6±4.4 to 95.8±5.8 bpm (n=11, P<0.03) respectively. 5-HT (1μM and 10μM) caused sinoatrial tachycardia that did not fade (n=8) and was not affected by Cil and Rol or their combination and –logEC50M values for 5-HT, estimated from concentration-effect curves (n=5-8 per group), were not different. 5-HT (10μM), incubated for 2 min, increased left atrial force from 4.2±1.5 to 8.4±2.3 mN (n=8, P<0.003) by the 20th min the response faded to 5.4±1.4 mN. In the presence of Cil or Rol 5-HT increased force from 5.5±0.8 to 14.0±2.1 (n=12, P<0.0001) and 4.8±0.6 to 12.9±1.3 mN (n=8, P<0.0001) respectively at the 2nd min; these responses partially faded to 9.0±1.5 and 9.3±1.2 mN respectively by the 20th min. The inotropic responses to (-)-isoprenaline (200μM) were not different in the absence (n=14) or presence of Cil (n=16) or Rol (n=18). Left atrial cAMP was determined by radioimmunoassay in freeze-clamped tissues (Table 1). 5-HT only increased cAMP by the 20th min. Both Rol and Cil caused increases of cAMP that faded. The (-)-isoprenaline-evoked cAMP response was markedly increased by Rol but not significantly by Cil.

 

Table 1. Effects of PDE inhibitors, 5-HT (10 µM) and (-)-isoprenaline (200 µM) (ISO), on left atrial cAMP levels (pmol.mg-1 protein, mean ± s.e.m.)

 

Control

5-HT

ISO

 

n

 

n 2 min

n 20 min

n

 

Basal

21

17.8±1.6

13

19.6±2.7

8

27.0±4.9a

21

27.3±2.1d

Rol

16

22.6±2.4

8

33.2±5.9b

7

25.6±2.

15

298.7±86.5c

Cil

20

20.4±2.7

8

28.7±4.4c

11

21.1±2.7

20

31.2±4.2a

Rol+Cil

19

18.6±0.9

8

32.3±3.9d

8

26.6±4.0d

18

275.9±31.3d

aP<0.05, bP=0.058, cP<0.005, dP<0.001; P values refer to comparisons with controls

 

Conclusions: 1. Both PDE3 and PDE4 appear to modulate basal sinoatrial rate but not 5-HT4 receptor-mediated tachycardia. 2. Both PDE3 and PDE4 partially contribute to the fade of left atrial inotropic responses to 5-HT. 3. Some inconsistencies between cAMP data and inotropy of 5-HT reflect still unknown compartmentalization. 4. 5-HT4 receptors signal to both PDE3 and PDE4 while β-adrenoceptors signal mainly to PDE4.

 

De Maeyer, J.H. et al. (2006) Br. J. Pharmacol., 147, 140-157.
Kaumann, A.J. & Levy, F.O. (2006). Br. J. Pharmacol ., 147, 128-130