Beta-Adrenergic Receptors and Adenylate Cyclase Activity in Heart, Muscle and Adipose Tissue of German Landrace Pigs Selectively Bred for Differences in Backfat Deposition

Summary

The density of beta-adrenergic receptors was examined in heart, muscle (m. longissimus dorsi) and backfat tissue of German Landrace pigs using (-)-[³H]-dihydroalprenolol. The animals were selected for high (E⁺-line) or low (E^--line) activity of lipogenic enzymes in backfat and for low backfat thickness (U^--line) based on ultrasonic measurement. An unselected control (K⁰-line) also exists. The correlated selection response on stress susceptibility (measured by halothane reaction and by meat quality parameters) showed the following sequence of selection lines: U^- > E^- > E⁺ > K⁰.

The strains used for examination descended from the 9th generation (heart and muscle tissue) and the 10th generation (heart, muscle and adipose tissue) of this selection experiment. No further selection took place between the 9th and 10th generation.

Basal adenylate cyclase activity was determined as well as (-)-Isoproterenol + GTP or sodium fluoride (NaF) stimulated activity in heart and muscle of the 10th generation measuring [³H]-cAMP formation from [³H]-ATP. A good correlation (r ≥ 0.59, n = 9) between beta-adrenergic receptor density and adenylate cyclase activity was found which pointed to an efficient receptor-effector system. The number of beta-adrenergic receptors and adenylate cyclase activity was higher in the U^--line in all investigated tissues compared to the other lines in the 9th and 10th generation. Most differences between these lines were also significant. The succession of beta-adrenergic receptor density and adenylate cyclase activity was as followed: heart: U^- > E^- > E⁺ > K⁰ (9th generation) and U^- > E^- K⁰ > E⁺ (10th generation), m. longissimus dorsi: U^- > E⁺ > E^- > K⁰ (9th generation) and U^- > K⁰ > E⁺ > E^- (10th generation), adipose tissue: U^- > E^- > K⁰ > E⁺. This suggests that a higher number of beta-adrenergic receptors associated with a higher adenylate cyclase activity plays a part in stress-susceptibility of pigs. The raised density of beta-adrenergic receptors may be involved in accelerated muscle glycogenolysis (poor meat quality) and increased lipolysis (low Caracas fat) and may explain the known connection between these factors.

Key words

Beta-Adrenergic Receptors - Adenylate Cyclase - German Landrace Pigs - Heart Membranes - Muscle Membranes - Adipocyte Membranes - Meat Quality