Hypertensive state, independent of hypertrophy, exhibits an attenuated decrease in systolic function on cardiac kappa-opioid receptor stimulation

Abstract

Opioids/opiates are commonly administered to alleviate pain, unload the heart, or decrease breathlessness in patients with advanced heart failure. As such, it is important to evaluate whether the myocardial opioidergic system is altered in cardiac disease. A hamster model of spontaneous hypertension was investigated before the development of hypertension (1 mo of age) and in the hypertensive state (10 mo of age) to evaluate the effect of prolonged hypertension on myocardial opioidergic activity. Plasma beta-endorphin was decreased before the development of hypertension and in the hypertensive state (P < 0.05). There was no change in cardiac beta-endorphin content at either time point. No differences were detected in cardiac or plasma dynorphin A, Met-enkephalin, or Leu-enkephalin, or in cardiac peptide expression of kappa- or delta-opioid receptors. mu-Opioid receptor was not detected in either model. To determine how hypertension affects myocardial opioid signaling, the ex vivo work-performing heart was used to assess the cardiac response to opioid administration in healthy hearts and those subjected to chronic hypertension. Agonists selective for the kappa- and delta-opioid receptors, but not mu-opioid receptors, induced a concentration-dependent decrease in cardiac function. The decrease in left ventricular systolic pressure on administration of the kappa-opioid receptor-selective agonist, U50488H, was attenuated in hearts from hamsters subjected to chronic, untreated hypertension (P < 0.05) compared with control. These results show that peripheral and myocardial opioid expression and signaling are altered in hypertension.

Fig. 1.

Systolic effect of opioid receptor-selective agonist administration. Increasing concentrations of receptor-selective agonists decreased left ventricular systolic pressure (LVSP) in a concentration-dependent manner in spontaneously hypertensive (H4; □ and dashed lines) and control (⧫ and solid lines) hamster hearts at 1 and 10 mo. Agonists were administered for 10 min each in a cumulative concentration-response curve, and then drugs were washed out for 20 min. Agonists used were U50488H (10 nM–2 μM; A and B) for κ-opioid receptor (KOR); SNC80 (1 nM–3 μM; C and D) for δ-opioid receptor (DOR), and fentanyl (100 pM–10 μM; E and F) for μ-opioid receptor (MOR). Data are means ± SE; n = 4 hamster hearts. *P < 0.05 compared with control hamster heart.

Fig. 2.

Negative diastolic effect of opioid receptor-selective agonist administration. Increasing concentrations of receptor-selective agonists decreased left ventricular end-diastolic pressure (LVEDP) in a concentration-dependent manner in H4 (□ and dashed lines) and control (⧫ and solid lines) hamster hearts at 1 and 10 mo. Agonists were administered for 10 min each in a cumulative concentration-response curve; drugs were then washed out for 20 min. Agonists used were U50488H (10 nM–2 μM; A and B) for KOR, SNC80 (1 nM-3-μM; C and D) for DOR, and fentanyl (100 pM–10 μM; E and F) for MOR. Data are means ± SE; n = 4 hamster hearts.

Fig. 3.

Receptor-selective antagonists inhibit opioid agonist effects. The KOR-selective agonist nor-binaltorphimine (nor-BNI; 5 μM) was administered for 15 min before 10-min infusion of U50488H (500 nM) in H4 (□ and dashed lines) and control (⧫ and solid lines) hamster hearts at 10 mo, and LVSP (A) and LVEDP (C) were measured. The DOR-selective antagonist naltrindole (Nal; 500 nM) was administered for 10 min before 10 min SNC80 (500 nM) infusion in H4 and control hamsters hearts at 10 mo, and LVSP (B) and LVEDP (D) were measured. Data are means ± SE; n = 3 hamster hearts.

Fig. 4.

Plasma concentrations of endogenous opioid peptides. Plasma concentration of β-endorphin (βe; A), dynorphin A (DA; B), Leu-enkephalin (LE; C), and Met-enkephalin (ME; D) in H4 (open bars) and control (black bars) hamsters at 1 and 10 mo. Peptide concentrations were determined by a competitive immunoassay and normalized on a per-assay basis to a standard control and to mg protein. Data are means ± SE; n = 10–15 hamsters. *P < 0.05 compared with age-matched control; #P < 0.05 compared with 1-mo cohort.

Fig. 5.

Cardiac opioid receptor (OR) levels. ORs were detected by Western blot using antibodies for the KOR, DOR, and MOR in left ventricles from H4 (open bars) and control (black bars) hamsters following prolonged hypertension. Opioid receptor levels were normalized to GAPDH levels on the same membrane. Top: representative Western blots for each receptor. AU, arbitrary units; n = 5 hamster hearts.

Fig. 6.

Inhibition of cAMP accumulation by opioid receptor-selective agonists. The KOR agonist U50488H (100 μM) and DOR agonist SNC80 (100 μM) inhibited the isoproterenol-induced accumulation of cAMP in left ventricular cardiomyocytes from normotensive control (black bars) and hypertensive (open bars) hamsters. Agonist effects were blocked by the antagonists nor-BNI (NOR; 100 μM, KOR inhibitor) and naloxone (NLX; 100 μM, nonselective OR blocker). Data are represented as percentage of isoproterenol alone. Data are means ± SE; n = 7 hamsters. *P < 0.05 compared with agonist treatment alone. #P < 0.05 compared with isoproterenol treatment alone.