Potentiation by insulin and insulin-like growth factor-1 of glibenclamide-sensitive K+ currents in follicle-enclosed Xenopus oocytes

The endogenous cannabinoid anandamide inhibits cromakalim-activated K+ currents in follicle-enclosed Xenopus oocytes

The effect of the endogenous cannabinoid anandamide on K(+) currents activated by the ATP-sensitive potassium (K(ATP)) channel opener cromakalim was investigated in follicle-enclosed Xenopus oocytes using the two-electrode voltage-clamp technique. Anandamide (1-90 microM) reversibly inhibited cromakalim-induced K(+) currents, with an IC(50) value of 8.1 +/- 2 microM. Inhibition was noncompetitive and independent of membrane potential. Coapplication of anandamide with the cannabinoid type 1 (CB(1)) receptor antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboximide hydrochloride (SR 141716A) (1 microM), the CB(2) receptor antagonist N-[(1S)endo-1,3,3-trimethyl bicyclo heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)-pyrazole-3-carboxamide (SR144528) (1 microM), or pertussis toxin (5 microg/ml) did not alter the inhibitory effect of anandamide, suggesting that known cannabinoid receptors are not involved in anandamide inhibition of K(+) currents. Similarly, neither the amidohydrolase inhibitor phenylmethylsulfonyl fluoride (0.2 mM) nor the cyclooxygenase inhibitor indomethacin (5 microM) affected anandamide inhibition of K(+) currents, suggesting that the effects of anandamide are not mediated by its metabolic products. In radioligand binding studies, anandamide inhibited the specific binding of the K(ATP) ligand [(3)H]glibenclamide in the oocyte microsomal fractions, with an IC(50) value of 6.3 +/- 0.4 microM. Gonadotropin-induced oocyte maturation and the cromakalim-acceleration of progesterone-induced oocyte maturation were significantly inhibited in the presence of 10 microM anandamide. Collectively, these results indicate that cromakalim-activated K(+) currents in follicular cells of Xenopus oocytes are modulated by anandamide via a cannabinoid receptor-independent mechanism and that the inhibition of these channels by anandamide alters the responsiveness of oocytes to gonadotropin and progesterone.

Dual effect of insulin-like growth factor on the apical 70-pS K channel in the thick ascending limb of rat kidney

We used the patch-clamp technique to study the effect of insulin-like growth factor I (IGF-I) on the apical 70-pS K channel in the isolated thick ascending limb (TAL) of the rat kidney. The isolated TAL was cut open to gain access to the apical membrane. Addition of 25 nM IGF-I stimulates the apical 70-pS K channel and increases channel activity, defined by the product of channel open probability and channel number, from 0.31 to 1.21. The stimulatory effect of IGF-I is not mediated by nitric oxide- or protein tyrosine phosphatase-dependent mechanisms, because inhibition of nitric oxide synthase or blocking protein tyrosine phosphatase did not abolish the stimulatory effect of IGF-I on the 70-pS K channel. In contrast, inhibition of mitogen-activated protein (MAP) kinase with PD-98059 or U0126 abolished the stimulatory effect of IGF-I. This suggests that MAP kinase is responsible for mediating the effect of IGF-I on the apical K channels. Moreover, the effect of IGF-I on the apical 70-pS K channel is biphasic because high concentrations (>200 nM) inhibit apical 70-pS K channels. Application of 400 nM IGF-I decreased channel activity from 1.45 to 0.2. The inhibitory effect of IGF-I is not blocked by calphostin C (an inhibitor of PKC), but inhibition of protein tyrosine kinase with herbimycin A abolished the IGF-induced inhibition. We conclude that IGF-I has a dual effect on the apical 70-pS K channel in the TAL: low concentrations of IGF-I stimulate, whereas high concentrations inhibit the channel activity. The stimulatory effect of IGF-I is mediated by a MAP kinase-dependent pathway, whereas the inhibitory effect is the result of stimulation of protein tyrosine kinase.