INOSITOL 1,4,5-TRISPHOSPHATE MASS CONTENT IN ISOLATED-PERFUSED RAT-HEART DURING ALPHA-1-ADRENOCEPTOR STIMULATION

Inositol-1,4,5-trisphosphate (IP3) has been proposed to be a second me ssenger in response to alpha-1-adrenoceptor stimulation also in myocar dial cells. We studied the effect of alpha-1-adrenoceptor stimulation (5 x 10(-5) mol/l phenylephrine or 5 x 10(-5) mol/l noradrenaline both in the presence of 10(-6) mol/l timolol) on IP3 mass content in isola ted perfused rat hearts. IP3 content was determined by a specific rece ptor-binding assay-kit (TRK 1000, Amersham) after validating the metho d. For comparison also the effect of muscarinic stimulation (10(-4) mo l/l carbachol in the presence of 10(-6) mol/l timolol) on IP3 content was measured in corresponding preparations. A basal IP3 level of about 75 pmol/mg protein was found. There were no prominent effects of alph a-1-adrenoceptor stimulation on total IP3 content in isolated perfused rat hearts. Phenylephrine gave a statistically significant increase o f about 40% at 1/4 min and a statistically significant decrease of abo ut 25% at 4 min after start of exposure. Noradrenaline, however, gave no statistically significant change of IP3 at the time-points studied. Muscarinic stimulation caused a slight, statistically insignificant, increase of IP3 at 1/4 min. The results are compatible with an assumpt ion that agonist stimulation evokes a localized increase of IP3 which may be masked by a relatively high total IP3 mass content. The IP3 pea k after phenylephrine coincided with the early positive inotropic phas e of the response reported earlier in perfused rat hearts for alpha-1- adrenoceptor stimulation by phenylephrine. Although this might be comp atible with a role for IP3 in this early and transient phase, a mediat or function of IP3 in the inotropic response is not established.