Animal experiment:

In juvenile rats of 100 ± 8 g body weight (bw), CHF is induced by supracoronary aortic banding. In brief, rats are anesthetized by intraperitoneal injection of ketamine (87 mg/kg bw) and xylazine (13 mg/kg bw). Rats are placed in the supine position, the chest wall is shaved, and a left thoracotomy is performed in the third intercostal space during ventilation with 100% O2. The ascending aorta is freed from connective tissue and partially occluded by implantation of a titanium clip with a defined internal diameter of 0.8 mm. After surgical closure of the thorax, the rats are allowed to recover from anesthesia. For postoperative analgesia, rats receive 250 mg/kg bw of metamizole intramuscularly immediately after the operation and on the first postoperative day. Sham-operated rats serve as controls. After recovery from anesthesia, the animals are placed in cages with free access to water and standard laboratory diet. For inhalation, milrinone (0.2-5 mg/mL) or NaCl (0.9%) are nebulized using an ultrasonic nebulizer and inhaled for 3 min at identical peak inspiratory pressures as used throughout the experiment. A 3-min nebulization of 1 mg/mL milrinone results in vaporization of 14 μg of the phosphodiesterase-3 inhibitor as determined by microgravimetry. Therefore, the respective dose of 39 μg/kg is analog to inhaled doses in human studies. For intravenous delivery, milrinone (initial bolus of 2-10 μg/kg, followed by 0.2-1 μg/kg/min) or equivalent volumes of NaCl (0.9%; initial bolus of 1.6 mL/kg, followed by 10 μL/kg/h) are administered by an infusion pump for 10 min.

Milrinone is a selective inhibitor of phosphodiesterase 3 (PDE-3) with a IC50 value of 56±12nM [1]

Milrinone has been noted to inhibit both members of the PDE3 family, PDE3A, which is prominent in heart, vascular smooth muscle, and platelets, as well as PDE3B, which is prominent in fat, liver, and pancreatic islets. Milrinone has shown the effect of concentration-dependent inhibition of PDE3 on the photolabelling with a IC50 value of 56±12nM. In addition, Milrinone has been reported to increase the accumulation of [3H]cAMP with a EC50 value of 5329±970nM in platelets. Apart from these, Milrinone has been revealed to have positive inotropic and vasodilatiry activities and thus used in clinical practice for short-trem treatment of patients who have the acute decompensated heart failure [1, 2].

References:
[1] Tang KM1, Jang EK, Haslam RJ. Photoaffinity labelling of cyclic GMP-inhibited phosphodiesterase (PDE III) in human and rat platelets and rat tissues: effects of phosphodiesterase inhibitors. Eur J Pharmacol. 1994 Jun 15;268(1):105-14.
[2] Cheung P1, Yang G, Boden G. Milrinone, a selective phosphodiesterase 3 inhibitor, stimulates lipolysis, endogenous glucose production, and insulin secretion. Metabolism. 2003 Nov;52(11):1496-500.