Ghrelin-evoked endothelium-dependent vasodilatation is preserved in early stage streptozotocin diabetes

Banafsheh Moazed, Urmila Shinde & Venkat Gopalakrishnan, Department of Pharmacology, University of Saskatchewan, Saskatoon SK S7N 5E5, Canada.

We have shown that ghrelin promotes endothelium-dependent but nitric oxide (NO) and cyclooxygenase-independent vasodilator responses in phenylephrine (PE) constricted perfused rat mesenteric vascular bed (MVB). This is mediated via selective activation of endothelium-derived hyperpolarization factor (EDHF)/process (1). Ghrelin-evoked hypotension was attenuated by preinfusion of a combination of apamin and charybdotoxin, which blocks EDHF (2). It is known that endothelium-mediated, NO and EDHF-dependent vasodilator responses to acetylcholine (Ah) is blunted in the MVB of Type-1 model of streptozotocin (STZ) diabetic rats. Plasma ghrelin level is also elevated in STZ-diabetic rats. Therefore, the present study aims to compare the vasodilator responses to ghrelin (which recruits only EDHF) vs. ACh (which recruits both NO and EDHF) in the MVB of STZ rats.

12-weeks old male Sprague-Dawley rats were made diabetic with a single intraperitoneal injection of STZ (65 mg/kg). Experiments were carried out at 4 and 8 weeks after STZ treatment (3). Blood samples were collected from both fed and overnight fasted rats (n>6) for analysis of plasma ghrelin. The responses to ghrelin and ACh were determined in PE constricted perfused MVB maintained at a constant flow rate of 5ml/min (3).

Plasma ghrelin levels were significantly higher in fasted control (P<0.05) and diabetic (P<0.01) rats compared to their fed counterparts. Diabetic rats exhibited higher plasma ghrelin levels at 4 (P<0.05) and 8 (P<0.01) weeks compared to their respective control groups. The plasma ghrelin levels increased with the progression of diabetic state of the animals. Ghrelin (10 pM) evoked 40-45% relaxation in PE constricted MVB of fed control rats whereas this was much lower (20-23%; P<0.01) in fasted rats while the responses to ACh remained unaffected in both fasted and fed rats (Table 1). There was duration-dependent diminution in ACh-evoked vasodilator responses whereas the responses to ghrelin were diminished only at 8 weeks after STZ treatment.

These data suggest that the elevated plasma ghrelin levels may result in reduced vasodilator responses in the MVB. However, the diabetic state per se may not affect EDHF-dependent vasodilator responses at least in the early stages of STZ diabetes whereas the responses to ACh predominantly mediated by NO, is progressively diminished right from the early stages of endothelial dysfunction in STZ diabetic rats.

Table. Plasma Ghrelin levels and MVB Responses to Ghrelin and ACh in STZ-Diabetic Rats

*P < 0.05, **P < 0.01 fed v/s fasted (control v/s control; diabetic v/s diabetic) rats.
P < 0.05, $$P < 0.01 fed v/s fed and fasted v/s fasted (control v/s diabetic) rats.

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