Insulin-like growth factor I (IGF-I) acts through the IGF-I receptor tyrosine kinase to stimulate cell proliferation, transformation and cellular protection. IGF-I plays a role in the stimulation of vascular smooth muscle cell growth (Sowers, 1997), and also has rapid vasoactive effects (Oltman et al. 2000). We have found that IGF-I inhibits arterial K_ATP currents, and investigated this using cells isolated enzymatically from small mesenteric arteries obtained from adult Wistar rats which had been killed humanely in accordance with Schedule 1. We used patch-clamp to record whole-cell K_ATP currents at -60 mV in symmetrical 140 mM K⁺ solutions. Pinacidil (10 µM) activated a glibenclamide-sensitive K_ATP current, and IGF-I (10 nM) inhibited this current by 24 ± 2 % (mean ± S.E.M., n = 9 cells).

The inhibitory effect of IGF-I was abolished in the presence of the tyrosine kinase inhibitor genistein (10 µM), consistent with a requirement for tyrosine phosphorylation. Inhibition of possible downstream consequences of tyrosine phosphorylation that involve mitogen-activated protein kinase or phospholipase C, with PD 98059 (10 µM) or U 73122 (10 µM), respectively, did not affect K_ATP current inhibition. In contrast, the inhibitors of phosphoinositide 3-kinase (PI 3-kinase), wortmannin (10 nM) or LY 294002 (10 µM), greatly attenuated K_ATP inhibition by IGF-I, reducing it to 3 ± 2 and 5 ± 2 %, respectively (P < 0.001 vs. control, ANOVA followed by Student-Newmann-Keuls test).

Possible signalling pathways downstream from PI 3-kinase include those involving protein kinase C, protein kinase A, or p70 ribosomal protein-S6 kinase. However, inhibitors of these pathways (Ro 31-8220, 1 µM; intracellular PKA inhibitor peptide 5-24, 5 µM; rapamycin, 50 nM) did not prevent inhibition of K_ATP current by IGF-I. K_ATP currents were larger in cells dialysed with intracellular solution that contained phosphatidylinositol 4, 5-bisphosphate (PIP₂, 5 µM) than in cells dialysed with control solution; currents normalised to cell capacitance were 12.3 ± 1.8 and 7.2 ± 0.5 pA pF^-1 in PIP₂ and control cells, respectively (P < 0.01). PIP₂ also nearly abolished the inhibitory effect of IGF-I on K_ATP current. The effect of intracellular PIP₃ was very similar to that of PIP₂.

Our results suggest that the inhibitory action of IGF-I on K_ATP channels occurs through PI 3-kinase, possibly through a reduction in the concentration of PIP₂ local to the channel.

We thank The Wellcome Trust and the British Heart Foundation for support.