Intrinsic cardiac muscle function, calcium handling and beta-adrenergic responsiveness is impaired in rats with growth hormone deficiency

To evaluate whether growth hormone (GH) is required for normal cardiac musc le function, we studied left ventricular papillary muscles of mutant GH-def icient rats, Developed tension normalized by cross-sectional area (DT), int racellular [Ca2+](i) (aequorin method) and beta-adrenergic responsiveness w ere assessed with or without 3 weeks GH replacement therapy and compared to normal controls. Steady-state force-Ca2+ relationship was determined in te tanized ryanodine-treated muscles. beta-adrenergic responsiveness was teste d during graded isoproterenol stimulation, [Ca2+](i) at baseline and the EC 50 of the force-Ca2+ relationship were similar in all groups. In dwarf rats , DT at baseline was reduced by 43% compared to controls, due to a decrease d maximal Ca2+-activated force. beta-adrenergic responsiveness of systolic Ca2+-release and mechanical function were depressed in dwarf rats. GH treat ment caused at least partial improvement of the depressed parameters. These data support the hypothesis that GH is required for normal intrinsic funct ion of cardiac muscle by maintaining Ca2+- and beta-adrenergic responsivene ss. (C) 1999 Harcourt Publishers Ltd.