Growth hormone (GH) exerts its actions via coordinated pulsatile secretion from a GH cell network into the bloodstream. Practically nothing is known about how the network receives its inputs in vivo and releases hormones into pituitary capillaries to shape GH pulses. Here we have developed in vivo approaches to measure local blood flow, oxygen partial pressure, and cell activity at single-cell resolution in mouse pituitary glands in situ. When secretagogue (GHRH) distribution was modeled with fluorescent markers injected into either the bloodstream or the nearby intercapillary space, a restricted distribution gradient evolved within the pituitary parenchyma. Injection of GHRH led to stimulation of both GH cell network activities and GH secretion, which was temporally associated with increases in blood flow rates and oxygen supply by capillaries, as well as oxygen consumption. Moreover, we observed a time-limiting step for hormone output at the perivascular level; macromolecules injected into the extracellular parenchyma moved rapidly to the perivascular space, but were then cleared more slowly in a size-dependent manner into capillary blood. Our findings suggest that GH pulse generation is not simply a GH cell network response, but is shaped by a tissue microenvironment context involving a functional association between the GH cell network activity and fluid microcirculation.blood flow | hormone pulsatility | oxygen pressure | tissue microenvironment | extracellular space

There are well-recognized sex differences in many pituitary endocrine axes, usually thought to be generated by gonadal steroid imprinting of the neuroendocrine hypothalamus. However, the recognition that growth hormone (GH) cells are arranged in functionally organized networks raises the possibility that the responses of the network are different in males and females. We studied this by directly monitoring the calcium responses to an identical GH-releasing hormone (GHRH) stimulus in populations of individual GH cells in slices taken from male and female murine GH-eGFP pituitary glands. We found that the GH cell network responses are sexually dimorphic, with a higher proportion of responding cells in males than in females, correlated with greater GH release from male slices. Repetitive waves of calcium spiking activity were triggered by GHRH in some males, but were never observed in females. This was not due to a permanent difference in the network architecture between male and female mice; rather, the sex difference in the proportions of GH cells responding to GHRH were switched by postpubertal gonadectomy and reversed with hormone replacements, suggesting that the network responses are dynamically regulated in adulthood by gonadal steroids. Thus, the pituitary gland contributes to the sexually dimorphic patterns of GH secretion that play an important role in differences in growth and metabolism between the sexes. sex hormones | body growth | calcium signaling | systems biology I n most species, males and females display a marked phenotypic divergence in body size, with increased growth rate and body mass being a predominantly masculine trait. Furthermore, in all species examined to date, the growth hormone (GH) axis demonstrates sex-specific differences in hormone contents, secretory outputs, and secretory patterns (1) and their effects on gene expression (2-4). The secretion of GH is controlled by hypothalamic GH-releasing hormone (GHRH) and somatostatin, and there is good evidence for sex-specific imprinting on hypothalamic hypophysiotropic neurons exerted by gonadal steroid exposure early in life (5), with ongoing effects during puberty (6). This has led to the conclusion that the sexually dimorphic control of GH patterns reflects sex differences in GHRH and somatostatin inputs to the pituitary gland. Acute changes in gonadal steroid environment drastically alter the patterns of GH pulsatility in adulthood (7,8); however, although they receive sexually dimorphic inputs (9, 10), GHRH neurons do not display sex-specific electrical characteristics (9, 11). We have previously shown that GH cells in the male mouse pituitary gland form an extensive homotypic cell network with an architecture that exhibits marked plasticity during sexual maturation and that can be altered by gonadectomy (12). Thus, it was important to determine whether male and female pituitary glands would show different responses to the same stimulus in the absence of any hypothalamic influence. To explore this, we assessed the functional activit...