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ABSTRACT. Phototherapy of newborn rats (NBR) resulted in a decrease in serum calcium and melatonin levels. Transcranial light penetrance in NBR increased with wavelength. Below 640 nm (penetrance = 6.9%), no hypocalcemic effect could be demonstrated. Shielding the occiput of NBR prevented a decrease in serum calcium during phototherapy and substantially reduced the decrease in melatonin found in unshielded NBR. Intraperitoneal injection of propranolol, an inhibitor of melatonin synthesis, caused a decrease in serum calcium in shaded NBR. In contrast, when melatonin was injected with propranolol a decrease in serum calcium did not occur. Additionally, intraperitoneal isoproterenol before phototherapy protected against a decrease in serum calcium. These data are consistent with an hypothesis that a decrease in serum calcium during phototherapy results from transcranial photic inhibition of melatonin synthesis. (Pediatr Res 22: [414][415][416]1987) Abbreviations NBR, newborn rats IP, intraperitoneal CAP, occipital shield Serum calcium concentration decreases in human infants and NBR exposed to white light at the energy level used in phototherapy for hyperbilirubinemia (5 pW/cm2/nm) (I, 2). This effect is not seen when the occiput is shielded (2), when blue light is used (2, 3), or, in NBR, when exogenous melatonin is given before light exposure (2). Transcranial illumination of the pineal inhibits melatonin synthesis (4), and we have hypothesized that the resultant decrease in serum melatonin might be responsible for the hypocalcemic effect of phototherapy (2).If this hypothesis is valid, it should be possible to demonstrate: 1) a relationship between the transcranial penetrance of light and its effect on calcium, 2) decreased serum melatonin concurrent with decreased serum calcium during phototherapy, and 3) changes in serum calcium after pharmacologic manipulation of melatonin synthesis.We determined transcranial light penetrance and serum calcium levels after exposure to light at 510 nm (blue), 545 nm (green), and 640 nm (yellow). We also measured serum melatonin and calcium in NBR during phototherapy with and without CAP; controls were shaded littermates.

The concentration of melatonin was determined in serum samples obtained at 15-min intervals during a 4-h period from seven normal girls, 9.5-16.5 yr of age, and eight normal boys, 9.0-16.8 yr of age. All samples were obtained between 0800 and 1300 h. An episodic pattern of melatonin secretion was found in all subjects. There was no significant (P greater than 0.1) difference between girls and boys as to number of episodes [3.1 +/- 0.4 (+/- SD) vs. 3.4 +/- 0.5 per 4 h), absolute melatonin increase (49.0 +/- 6.7 vs. 50.4 +/- 4.1 pg/ml), or apparent melatonin half-life (20.3 +/- 0.8 vs. 20.8 +/- 1.0 min). The mean absolute increase during secretory episodes correlated (r = 0.843; P less than 0.005) with the mean melatonin concentration per 4 h, and a significant (P less than 0.05) inverse correlation (r = -0.508) was found between the mean percent increment of the melatonin secretory episode and the mean apparent melatonin half-life. A significant correlation between melatonin secretion and previously reported (22) episodic LH secretion was not found. In girls and boys, the mean melatonin concentration increased with advancing puberty. The increase in melatonin concentration was due to an increase in the amplitude of secretory episodes. These data are consistent with an influence of gonadal function on melatonin secretion.

The concentrations of LH and FSH were determined in serum samples obtained at 15 min intervals during a 4 h period from seven normal girls, 9.5-16.5 years of age, and eight normal boys, 9.0-16.8 years of age. An episodic pattern of LH secretion was found in all subjects. There was no significant (P greater than 0.1) difference between girls and boys as to number of episodes (3.0 +/- 0.6 vs. 3.0 +/- 0.0 per 4 h), absolute LH increase (2.6 +/- 1.1 vs. 2.4 +/- 1.0 mIU per ml) or "apparent LH half-life" (64.3 +/- 20.8 vs. 76.1 +/- 23.4 min). Mean absolute increase during secretory episodes correlated (R = 0.89, P less than 0.005) with the mean LH concentration per 4 h, and a significant (P less than 0.025) inverse correlation (R = -0.52) was found between the mean per cent increment of the LH secretory episode and the mean "apparent LH half-life." Minor fluctuations of FSH were also observed. In girls and boys, the mean concentration of both gonadotropins increased with advancing puberty. The increase in LH concentration was due to an increase in the amplitude of secretory episodes rather than to an increase in the number of episodes. The increase in FSH concentration reflected a consistent elevation throughout the period of sampling.

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